Ink composition, inkjet recording method, printed material, process for producing lithographic printing plate, and lithographic printing plate

ABSTRACT

An ink composition is provided that includes (a) an onium salt, a counteranion thereof having a volume of 1,000 Å 3  or greater and (b) a cationically polymerizable compound. There is also provided an inkjet recording method that includes (a′) a step of discharging the ink composition onto a recording medium and (b′) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation. There are also provided a printed material obtained by the inkjet recording method, a process for producing a lithographic printing plate employing the ink composition, and a lithographic printing plate obtained by the production process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink composition suitably used forinkjet recording, an inkjet recording method, and a printed materialemploying same; furthermore, it relates to a lithographic printing plateobtained using the ink, and a process for producing a lithographicprinting plate. More particularly, it relates to an ink compositionsuitable for inkjet recording that cures with high sensitivity uponexposure to radiation, can form a high quality image, and has goodstorage stability, an inkjet recording method, a printed materialemploying same, a lithographic printing plate obtained using the ink,and a process for producing a lithographic printing plate.

2. Description of the Related Art

With regard to an image recording method for forming an image on arecording medium such as paper based on an image data signal, there arean electrophotographic system, sublimation type and melt type thermaltransfer systems, an inkjet system, etc. In the electrophotographicsystem, a process of forming an electrostatic latent image on aphotosensitive drum by electrically charging and exposing is required,and the system is complicated; as a result, there is the problem thatthe production cost is high. With regard to the thermal transfer system,although the equipment is inexpensive, due to the use of an ink ribbonthere is the problem that the running cost is high and waste material isgenerated. On the other hand, with regard to the inkjet system, theequipment is inexpensive and, since an image is formed directly on arecording medium by discharging an ink only on a required image area,the ink can be used efficiently and the running cost is low.Furthermore, there is little noise and it is excellent as an imagerecording system.

An ink composition that can be cured by exposure to radiation such asultraviolet rays and, in particular, an inkjet recording ink (radiationcuring type inkjet recording ink) are required to have sufficiently highsensitivity and provide a high image quality. By achieving highersensitivity, a large number of benefits are provided, such as highcurability toward radiation, a reduction in power consumption, longerlifetime due to a decrease in the load on a radiation generator, andprevention of formation of low molecular weight material originatingfrom insufficient curing. Furthermore, higher sensitivity particularlyimproves the cure strength of an image formed using the ink compositionand, in particular, the inkjet recording ink, particularly for theformation of a lithographic printing plate, and high plate life can beobtained.

On the other hand, as a cationic polymerization initiator, compoundsdisclosed by inventions described in, for example, JP-A-6-184170,JP-A-9-202873, and JP-A-9-221652 (JP-A denotes a Japanese unexaminedpatent application publication) can be cited, and there is a desire foran ink composition in which a cationically polymerizable compound hashigh polymerization efficiency and the ink composition has excellentsensitivity toward radiation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention, which has been accomplishedunder the above-mentioned circumstances, to provide an ink compositionthat cures with high sensitivity by exposure to actinic radiation, canform a high quality image, has excellent adhesion to a recording medium,has good storage stability, and can prolong the life of an ink head, andan inkjet recording method employing the ink composition.

It is another object of the present invention to provide a printedmaterial and a lithographic printing plate obtained using an inkcomposition that has excellent storage stability and can be cured withhigh sensitivity by exposure to actinic radiation, and a process forproducing a lithographic printing plate.

The above-mentioned objects can be accomplished by (1), (6), and (8) to(10) below. (2) to (5) and (7), which are preferred embodiments, arealso shown below.

-   (1) An ink composition comprising (a) an onium salt, a counteranion    thereof having a volume of 1,000 Å³ or greater, and (b) a    cationically polymerizable compound,-   (2) the ink composition according to (1) above, wherein the ink    composition comprises a colorant,-   (3) the ink composition according to (2) above, wherein the colorant    is a pigment or an oil-soluble dye,-   (4) the ink composition according to (3) above, wherein the    oil-soluble dye has an oxidation potential of 1.0 V (vs SCE) or    greater,-   (5) the ink composition according to any one of (1) to (4) above,    wherein it is for inkjet recording,-   (6) an inkjet recording method comprising (a) a step of discharging    an ink composition onto a recording medium, and (b) a step of curing    the ink composition by irradiating the discharged ink composition    with actinic radiation, wherein the ink composition is the ink    composition according to any one of (1) to (5) above,-   (7) the inkjet recording method according to (6) above, wherein the    actinic radiation is ultraviolet radiation emitted by a light    emitting diode that has a light emission peak wavelength in the    range of 350 to 420 nm and generates ultraviolet radiation whose    maximum illumination intensity on the surface of a recording medium    is 10 to 2,000 mW/cm²,-   (8) a printed material recorded by the inkjet recording method    according to either (6) or (7),-   (9) a process for producing a lithographic printing plate, the    process comprising (a) a step of discharging the ink composition    according to any one of (1) to (5) above onto a hydrophilic support,    and (b) a step of curing the ink composition by irradiating the    discharged ink composition with actinic radiation so as to form a    hydrophobic image on the hydrophilic support from the cured ink    composition, and-   (10) a lithographic printing plate produced by the process for    producing a lithographic printing plate according to (9) above.

DETAILED DESCRIPTION OF THE INVENTION

The ink composition of the present invention (hereinafter, also simplycalled an ‘ink’) comprises (a) an onium salt, a counteranion thereofhaving a volume of 1,000 Å³ (cubic angstroms) or greater, and (b) acationically polymerizable compound.

The present invention is explained in detail below.

(1) Components Contained in Ink Composition

The ink composition of the present invention can be cured by radiation,and may comprise (a) an onium salt, a counteranion thereof having avolume of 1,000 Å³ or greater, and (b) a cationically polymerizablecompound, and as necessary (c) a colorant, (d) a sensitizing dye, (e) acosensitizer, (f) another polymerizable compound, (g) anotherpolymerization initiator, and (h) other components.

The ‘radiation’ referred to in the present invention is not particularlylimited as long as it is actinic radiation that can provide energy thatenables an initiating species to be generated in the ink compositionwhen irradiated, and broadly covers α rays, γ rays, X rays, ultravioletrays, visible light, and an electron beam; among these ultraviolet raysand an electron beam are preferable from the viewpoint of curingsensitivity and the availability of equipment, and ultraviolet rays areparticularly preferable. The ink composition of the present invention istherefore preferably an ink composition that can be cured by exposure toultraviolet rays as radiation.

(a) Onium Salt, a Counteranion Thereof Having a Volume of 1,000 Å³ orGreater

The ink composition of the present invention comprises (a) an oniumsalt, a counteranion thereof having a volume of 1,000 Å³ or greater(hereinafter, also called a ‘specific onium salt’).

The specific onium salt (a) of the ink composition of the presentinvention has the function of generating a cationic polymerizationactive species upon exposure to radiation, and initiating and makingpolymerization of (b) a cationically polymerizable compound proceed.

The specific onium salt (a) that can be used in the present inventionmay be used singly or in a combination of two or more types.

The counteranion of the specific onium salt is not particularly limitedas long as it has a volume of 1,000 Å³ or greater, and the counteranionis preferably an organoboron anion, organosulfonate anion,organocarboxylate anion, polymeric sulfonate (poly)anion, or polymericcarboxylate (poly)anion that has a volume of 1,000 Å³ or greater, morepreferably an organoboron anion, organosulfonate anion, ororganocarboxylate anion that has an volume of 1,000 Å³ or greater, andyet more preferably an organoboron anion having a volume of 1,000 Å³ orgreater. In the case of a polyanion, the volume of the counteranion isdefined as the quotient obtained by dividing the entire volume of thepolyanion by the number of anions.

The counteranion preferably contains in its structure at least oneelectron-withdrawing group, preferably contains at least oneperfluoroalkyl group or perfluoroaryl group, and particularly preferablycontains at least one perfluoroalkyl group. With regard to the positionat which the electron-withdrawing group is bonded to the counteranion,it may preferably be bonded directly to the anionic atom or bonded tothe anionic atom via an unsaturated bond or an aromatic ring.

The volume of the counteranion is 1,000 Å³ (1.00 nm³) or greater,preferably 1,200 Å³ or greater, and more preferably 1,500 Å³ or greater.

The volume of the counteranion is preferably no greater than 5.00×10³Å³, and more preferably no greater than 3.00×10³ Å³.

The volume of the counteranion referred to in the present inventiondenotes the volume occupied by a van der Waals sphere based on the vander Waals radius of atoms forming the counteranion.

A method for calculating the volume of a counteranion is notparticularly limited; a known method may be used, and examples thereofinclude a method in which a molecular structure is identified byradiation structural analysis of a crystal using X-ray structuralanalysis or neutron beam analysis and calculation is carried out, and amethod in which an optimized structure of a molecule is determined by asemi-empirical molecular orbital method, a non-empirical molecularorbital method, or a molecular force field method and calculation iscarried out. As a method for determining molecular volume from themolecular structure, a known method may be used, and examples thereofinclude a space lattice division convergence method and an atomic groupcontribution method.

As a specific method for calculating the volume of a counteranion,calculation is carried out as follows using WinMOPAC calculationsoftware (manufactured by Fujitsu).

The most stable conformation obtained by molecular force fieldcalculation (MM3) is subjected to molecular orbital calculation (AM1) togive a van der Waals surface, and the value of a surface volume (usingan accessible surface parameter) is determined.

Preferred specific examples of the counteranion having a volume of 1,000Å³ or greater include counteranions having the structures shown in An-1to An-12 below.

The onium ion (countercation) of the specific onium salt that can beused in the present invention may be an onium ion of a known onium salt;examples thereof include onium ions such as iodonium, sulfonium,phosphonium, diazonium, ammonium, pyridinium, quinolinium, acridinium,oxonium, selenonium, and arsonium, and among these, onium ions such asiodonium, sulfonium, phosphonium, diazonium, quinolinium, and acridiniumare preferable.

Examples of the countercation of the specific onium salt include cationssuch as onium ions of onium salts of group 15 to 17 elements describedin JP-A-6-184170, etc., diazonium ions of diazonium salts described inS. I. Schlesinger, Photogr. Sci. Eng., 18, 387 (1974), T. S. Bal et al.,Polymer, 21, 423 (1980), etc., ammonium ions of ammonium salts describedin U.S. Pat. Nos. 4,069,055, 4,069,056, and Re 27,992, JP-A-3-140140,etc., phosphonium ions of phosphonium salts described in D. C. Necker etal., Macromolecules, 17, 2468 (1984), C. S. Wen et al., The Proc. Conf.Rad. Curing ASIA, p. 478 Tokyo, Oct (1988), U.S. Pat. Nos. 4,069,055 and4,069,056, JP-A-9-202873, etc., iodonium ions of iodonium saltsdescribed in J. V. Crivello et al., Macromolecules, 10 (6), 1307 (1977),Chem. & Eng. News, Nov. 28, p. 31 (1988), European Patent Nos. 104,143,339,049, and 410,201, JP-A-2-150848, JP-A-2-296514, etc., sulfonium ionsof sulfonium salts described in J. V. Crivello et al., Polymer J.17,73(1985), J. V. Crivello et al., J. Org. Chem., 43, 3055 (1978), W. R.Watt et al., J. Polymer Sci., Polymer Chem. Ed., 22, 1789 (1984), J. V.Crivello et al., Polymer Bull., 14, 279 (1985), J. V. Crivello et al.,Macromolecules, 14(5), 1141 (1981), J. V. Crivello et al., J. PolymerSci., Polymer Chem. Ed., 17, 2877 (1979), European Patent Nos. 370,693,161,811, 410,201, 339,049, 233,567, 297,443, and 297,442, U.S. Pat. Nos.3,902,114, 4,933,377, 4,760,013, 4,734,444, and 2,833,827, German PatentNos. 2,904,626, 3,604,580, and 3,604,581, JP-A-7-28237, JP-A-8-27102,etc., quinolinium ions of quinolinium salts described in JP-A-9-221652,selenonium ions of selenonium salts described in J. V. Crivello et al.,Macromolecules, 10 (6), 1307 (1977), J. V. Crivello et al., J. PolymerSci., Polymer Chem. Ed., 17, 1047 (1979), etc., and arsonium ions ofarsonium salts described in C. S. Wen et al., The Proc. Conf. Rad.Curing ASIA, p. 478 Tokyo, Oct (1988), etc., but should not be construedas being limited thereto.

Furthermore, preferred examples of the countercation of (a) the specificonium salt include cations having structures represented by Formulae (I)to (VI) below.

In Formulae (I) to (VI) above, R¹ to R³ independently denote an arylgroup, R⁴ to R⁶ independently denote an alkyl group, an alkenyl group,an alkynyl group, an aryl group, a cyclic hydrocarbon group, or aheterocyclic group, R⁷ to R¹¹ independently denote an alkyl group, analkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbongroup, a heterocyclic group, an alkoxy group, or an aryloxy group, andR¹² to R¹⁷ independently denote a hydrogen atom, a halogen atom, or amonovalent organic group.

The alkyl groups denoted by R⁴ to R¹¹ preferably have 1 to 30 carbons,more preferably 1 to 20 carbons, and particularly preferably 1 to 8carbons, and may be a straight chain and may have a substituent.

The alkenyl groups denoted by R⁴ to R¹¹ preferably have 2 to 30 carbons,more preferably 2 to 20 carbons, and particularly preferably 2 to 8carbons, and may further have a substituent.

The alkynyl groups denoted by R⁴ to R¹¹ preferably have 2 to 30 carbons,more preferably 2 to 20 carbons, and particularly preferably 2 to 8carbons, and may further have a substituent.

The aryl groups denoted by R¹ to R¹¹ preferably have 6 to 30 carbons,more preferably 6 to 20 carbons, and particularly preferably 6 to 10carbons, and may further have a substituent.

The cyclic hydrocarbon groups denoted by R⁴ to R¹¹ preferably have 3 to30 carbons, more preferably 3 to 20 carbons, and particularly preferably3 to 10 carbons, and may further have a substituent.

The heterocyclic groups denoted by R⁴ to R¹¹ preferably have 4 to 30carbons, more preferably 4 to 20 carbons, and particularly preferably 4to 10 carbons, and may further have a substituent. Furthermore, thehetero atom contained in the heterocyclic group is preferably a nitrogenatom, an oxygen atom, or a sulfur atom.

The alkoxy groups denoted by R⁷ to R¹¹ preferably have 1 to 30 carbons,more preferably 1 to 20 carbons, and particularly preferably 1 to 8carbons. Furthermore, the alkoxy groups may have a substituent describedbelow, and the alkyl moiety of the alkoxy groups may be an alkenylgroup, an alkynyl group, a cyclic hydrocarbon group, or a heterocyclicgroup other than an aromatic group.

The aryloxy groups denoted by R⁷ to R¹¹ preferably have 6 to 30 carbons,more preferably 6 to 20 carbons, and particularly preferably 6 to 10carbons. Furthermore, the aryloxy groups may have a substituentdescribed below, and the aryl moiety of the aryloxy groups may be anaromatic heterocyclic group.

In Formula (II), R² and R³ may be bonded to each other to form a ring ifthis is possible.

In Formula (III), two or more of R⁴ to R⁶ may be bonded to each other toform a ring if this is possible.

In Formula (IV), two or more of R⁷ to R¹⁰ may be bonded to each other toform a ring if this is possible.

In Formula (V), two or more of R¹¹ to R¹⁴ may be bonded to each other toform a ring if this is possible.

In Formula (VI), two or more of R¹⁵ to R¹⁷ may be bonded to each otherto form a ring if this is possible.

With regard to substituents that may be possessed by the alkyl group,the alkenyl group, the alkynyl group, the aryl group, the hydrocarbongroup, the heterocyclic group, the alkoxy group, or the aryloxy group, amonovalent non-metallic atomic group other than hydrogen is used, andpreferred examples thereof include a halogen atom (—F, —Br, —Cl, —I), ahydroxyl group, an alkyl group, an aryl group, an alkenyl group, analkynyl group, an alkoxy group, an aryloxy group, a mercapto group, analkylthio group, an arylthio group, an alkyldithio group, an aryldithiogroup, an amino group, an N-alkylamino group, an N,N-dialkylamino group,an N-arylamino group, an N,N-diarylamino group, an N-alkyl-N-arylaminogroup, an acyloxy group, a carbamoyloxy group, an N-alkylcarbamoyloxygroup, an N-arylcarbamoyloxy group, an N,N-dialkylcarbamoyloxy group, anN,N-diarylcarbamoyloxy group, an N-alkyl-N-arylcarbamoyloxy group, analkylsulfoxy group, an arylsulfoxy group, an acylthio group,

an acylamino group, an N-alkylacylamino group, an N-arylacylamino group,a ureido group, an N′-alkylureido group, an N,N′-dialkylureido group, anN′-arylureido group, an N′,N′-diarylureido group, anN′-alkyl-N′-arylureido group, an N-alkylureido group, an N-arylureidogroup, an N′-alkyl-N-alkylureido group, an N′-alkyl-N-arylureido group,an N,N′-dialkyl-N-alkylureido group, an N,N′-dialkyl-N-arylureido group,an N′-aryl-N-alkylureido group, an N′-aryl-N-arylureido group, anN′,N′-diaryl-N-alkylureido group, an N,N′-diaryl-N-arylureido group, anN′-alkyl-N′-aryl-N-alkylureido group, an N′-alkyl-N′-aryl-N-arylureidogroup, an alkoxycarbonylamino group, an aryloxycarbonylamino group, anN-alkyl-N-alkoxycarbonylamino group, an N-alkyl-N-aryloxycarbonylaminogroup, an N-aryl-N-alkoxycarbonylamino group, anN-aryl-N-aryloxycarbonylamino group,

a formyl group, an acyl group, a carboxyl group and conjugate base groupthereof (hereinafter, called a carboxylate), an alkoxycarbonyl group, anaryloxycarbonyl group, a carbamoyl group, an N-alkylcarbamoyl group, anN,N-dialkylcarbamoyl group, an N-arylcarbamoyl group, anN,N-diarylcarbamoyl group, an N-alkyl-N-arylcarbamoyl group, analkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, anarylsulfonyl group, a sulfo group (—SO₃H) and conjugate base groupthereof (hereinafter, called a sulfonate group), an alkoxysulfonylgroup, an aryloxysulfonyl group, a sulfinamoyl group, anN-alkylsulfinamoyl group, an N,N-dialkylsulfinamoyl group, anN-arylsulfinamoyl group, an N,N-diarylsulfinamoyl group, anN-alkyl-N-arylsulfinamoyl group, a sulfamoyl group, an N-alkylsulfamoylgroup, an N, N-dialkylsulfamoyl group, an N-arylsulfamoyl group, anN,N-diarylsulfamoyl group, an N-alkyl-N-arylsulfamoyl group, anN-acylsulfamoyl group and conjugate base group thereof, anN-alkylsulfonylsulfamoyl group (—SO₂NHSO₂(alkyl)) and conjugate basegroup thereof, an N-arylsulfonylsulfamoyl group (—SO₂NHSO₂(aryl)) andconjugate base group thereof, an N-alkylsulfonylcarbamoyl group(—CONHSO₂(alkyl)) and conjugate base group thereof, anN-arylsulfonylcarbamoyl group (—CONHSO₂(aryl)) and conjugate base groupthereof,

a silyl group, an alkoxysilyl group (—Si(O-alkyl)₃), an aryloxysilylgroup (—Si(O-allyl)₃), a hydroxysilyl group (—Si(OH)₃) and conjugatebase group thereof, a phosphono group (—PO₃H₂) and conjugate base groupthereof (hereinafter, called a phosphonate group), a dialkylphosphonogroup (—PO₃(alkyl)₂), a diarylphosphono group (—PO₃(aryl)₂), analkylarylphosphono group (—PO₃(alkyl)(aryl)), a monoalkylphosphono group(—PO₃H(alkyl)) and conjugate base group thereof (hereinafter, called analkylphosphonate group), a monoarylphosphono group (—PO₃H(aryl)) andconjugate base group thereof (hereinafter, called an arylphosphonategroup), a phosphonooxy group (—OPO₃H₂) and conjugate base group thereof(hereinafter, called a phosphonatooxy group), a dialkylphosphonooxygroup (—OPO₃(alkyl)₂), a diarylphosphonooxy group (—OPO₃(aryl)₂), analkylarylphosphonooxy group (—OPO₃(alkyl)(aryl)), amonoalkylphosphonooxy group (—OPO₃H(alkyl)) and conjugate base groupthereof (hereinafter, called an alkylphosphonatooxy group), amonoarylphosphonooxy group (—OPO₃H(aryl)) and conjugate base groupthereof (hereinafter, called an arylphosphonatooxy group), a cyanogroup, and a nitro group. These substituents may be further substitutedwith the above-mentioned substituent, and may form a ring if this ispossible.

R¹² to R¹⁷ independently denote a hydrogen atom, a halogen atom, or amonovalent organic group.

Examples of the halogen atom denoted by R¹² to R¹⁷ include a fluorineatom, a chlorine atom, a bromine atom, and an iodine atom, and afluorine atom, a chlorine atom, or a bromine atom is preferable.

Examples of the monovalent organic group denoted by R¹² to R¹⁷ include ahydroxyl group, an alkyl group, an alkenyl group, an alkynyl group, anaryl group, a cyclic hydrocarbon group, a heterocyclic group, an alkoxygroup, an aryloxy group, an acyl group, an alkoxycarbonyl group, anacyloxy group, —SO₃—R^(a), —NR^(b)R_(c), a cyano group,—SiR^(d)R^(e)R^(f), —SOR⁹, —SO₂R^(g), and a nitro group; R^(a) denotes ahydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, anaryl group, an arylalkyl group, an alkali metal atom, or quaternaryammonium, R^(b), R^(c), and R^(g) independently denote an alkyl group,an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbongroup, or a heterocyclic group, and R^(d) to R^(f) independently denotean alkyl group, an alkenyl group, an alkynyl group, an aryl group, acyclic hydrocarbon group, a heterocyclic group, an alkoxy group, or anaryloxy group.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclichydrocarbon group, heterocyclic group, alkoxy group, and aryloxy groupdenoted by R¹² to R¹⁷ have the same meanings as those denoted by R⁷ toR¹¹ above, and preferred ranges are also the same. Furthermore, thesegroups may have the above-mentioned substituent.

The acyl group and the alkoxycarbonyl group denoted by R¹² to R¹⁷preferably have 1 to 30 carbons in the carbon chain side, andparticularly preferably 1 to 12, and may be a straight chain and mayhave the above-mentioned substituent.

The acyloxy group denoted by R¹² to R¹⁷ preferably has 1 to 30 carbons,and particularly preferably 1 to 12 carbons, and may be a straight chainand may have the above-mentioned substituent.

R^(a) in —SO₃—Ra denoted by R¹² to R¹⁷ is preferably a hydrogen atom,the above-mentioned alkyl group, which may have a substituent, theabove-mentioned aryl group, which may have a substituent, a lithiumatom, a sodium atom, or a potassium atom.

The above-mentioned alkyl group, alkenyl group, alkynyl group, arylgroup, cyclic hydrocarbon group, and heterocyclic group denoted by R^(b)and R^(c) in —NR^(b)R^(c) have the same meanings as those denoted by R⁷to R¹¹ above, and preferred ranges are also the same. Furthermore, thesegroups may have the above-mentioned substituent.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclichydrocarbon group, heterocyclic group, alkoxy group, and aryloxy groupdenoted by R^(d) to R^(f) in —SiR^(d)R^(e)R^(f) have the same meaningsas those denoted by R⁷ to R¹¹ above, and preferred ranges are also thesame. Furthermore, these groups may have the above-mentionedsubstituent.

The alkyl group, alkenyl group, alkynyl group, aryl group, cyclichydrocarbon group, and heterocyclic group denoted by R^(g) in —SOR^(g)or —SO₂R^(g) have the same meanings as those denoted by R⁷ to R¹¹ above,and preferred ranges are also the same. Furthermore, these groups mayhave the above-mentioned substituent.

Specific examples of the countercation in the specific onium salt (a)include countercations having the structures denoted by Ca-1 to Ca-41below.

With regard to the ink composition of the present invention, since theonium salt (a) contained therein has a counteranion having a volume of1,000 Å³ or greater, the nucleophilicity of the counteranion is low, theinhibition of a cationic polymerization growth reaction by thecounteranion is suppressed, the polymerization efficiency improves, andthe curing sensitivity increases.

Furthermore, when the counteranion of the specific onium salt (a)contains an electron-withdrawing group in its structure, compared with acompound containing no electron-withdrawing group, the nucleophilicityof the counteranion becomes lower, and the curing sensitivity improves.

A method for synthesizing the specific onium salt is not particularlylimited, but, for example, a counteranion having a volume of 1,000 Å³may be synthesized by a method described in J. Am. Chem. Soc., Vol. 127,p. 8958-8959 (2005), etc. Furthermore, various types of specific oniumsalts and starting materials therefor may be synthesized by a knownanionic exchange method described in, for example, JP-A-6-184170.

As the specific onium salt (a) that can be used in the presentinvention, an onium salt having in combination one anion selected fromthe group consisting of An-1 to An-12 above as a counteranion and onecation selected from the group consisting of Ca-1 to Ca-41 above as acountercation can be cited preferably.

(b) Cationically Polymerizable Compound

The cationically polymerizable compound used in the present invention isnot particularly limited as long as it is a compound that undergoes apolymerization reaction by virtue of an acid generated by the photo-acidgenerator , which will be described later, and is cured, and varioustypes of cationically polymerizable monomers known as photo-cationicallypolymerizable monomers may be used. Examples of the cationicallypolymerizable monomer include epoxy compounds, vinyl ether compounds,oxetane compounds described in JP-A-6-9714, JP-A-2001-31892,JP-A-2001-40068, JP-A-2001-55507, JP-A-2001-310938, JP-A-2001-310937,JP-A-2001 -220526, etc. As the cationically polymerizable compound, forexample, a cationically polymerizable type photocuring resin is known,and in recent years cationically photopolymerizable type photocuringresins sensitized to a visible light wavelength region of 400 nm orlonger have been disclosed in, for example, JP-A-6-43633 andJP-A-8-324137. As the radically polymerizable compound, photocuringmaterials employing photopolymerizable compositions described in, forexample, JP-A-7-159983, JP-B-7-31399, JP-A-8-224982, and JP-A-10-863 areknown.

Examples of the epoxy compounds include aromatic epoxides, alicyclicepoxides, and aliphatic epoxides, and examples of the aromatic epoxideinclude di- or polyglycidyl ethers produced by a reaction betweenepichlorohydrin and a polyhydric phenol having at least one aromaticnucleus or an alkylene oxide adduct thereof; specific examples includedi- or polyglycidyl ethers of bisphenol A or an alkylene oxide adductthereof, di- or polyglycidyl ethers of hydrogenated bisphenol A or analkylene oxide adduct thereof, and novolac type epoxy resins. Examplesof the alkylene oxide above include ethylene oxide and propylene oxide.

Examples of the alicyclic epoxides include cyclohexene oxide- andcyclopentene oxide-containing compounds obtained by epoxidizing acompound having at least one cycloalkene ring such as a cyclohexene ringor a cyclopentene ring with an appropriate oxidizing agent such ashydrogen peroxide or a peracid.

Examples of the aliphatic epoxides include di- or polyglycidyl ethers ofan aliphatic polyhydric alcohol or an alkylene oxide adduct thereof, andrepresentative examples thereof include diglycidyl ethers of an alkyleneglycol such as the diglycidyl ether of ethylene glycol, the diglycidylether of propylene glycol, and the diglycidyl ether of 1,6-hexanediol,polyglycidyl ethers of a polyhydric alcohol such as the di- ortriglycidyl ether of glycerol or an alkylene oxide adduct thereof, anddiglycidyl ethers of a polyalkylene glycol such as the diglycidyl etherof polyethylene glycol or an alkylene oxide adduct thereof and thediglycidyl ether of polypropylene glycol or an alkylene oxide adductthereof. Examples of the alkylene oxide above include ethylene oxide andpropylene oxide.

Detailed examples of monofunctional and polyfunctional epoxy compoundsthat can be used in the present invention are now given.

Examples of monofunctional epoxy compounds used in the present inventioninclude phenyl glycidyl ether, p-tert-butylphenyl glycidyl ether, butylglycidyl ether, 2-ethylhexyl glycidyl ether, allyl glycidyl ether,1,2-butylene oxide, 1,3-butadiene monooxide, 1,2-epoxydodecane,epichlorohydrin, 1,2-epoxydecane, styrene oxide, cyclohexene oxide,3-methacryloyloxymethylcyclohexene oxide, 3-acryloyloxymethylcyclohexeneoxide, and 3-vinylcyclohexene oxide.

Furthermore, examples of polyfunctional epoxy compounds includebisphenol A diglycidyl ether, bisphenol F diglycidyl ether, bisphenol Sdiglycidyl ether, brominated bisphenol A diglycidyl ether, brominatedbisphenol F diglycidyl ether, brominated bisphenol S diglycidyl ether,epoxy novolac resins, hydrogenated bisphenol A diglycidyl ether,hydrogenated bisphenol F diglycidyl ether, hydrogenated bisphenol Sdiglycidyl ether,3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexenecarboxylate,2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-meta-dioxane,bis(3,4-epoxycyclohexylmethyl) adipate, vinylcyclohexene dioxide,4-vinylepoxycyclohexane, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, 3,4-epoxy-6-methylcyclohexenyl3′,4′-epoxy-6′-methylcyclohexenecarboxylate,methylenebis(3,4-epoxycyclohexane), dicyclopentadiene diepoxide, thedi(3,4-epoxycyclohexylmethyl) ether of ethylene glycol, ethylenebis(3,4-epoxycyclohexanecarboxylate), dioctyl epoxyhexahydrophthalate,di-2-ethylhexyl epoxyhexahydrophthalate, 1,4-butanediol diglycidylether, 1,6-hexanediol diglycidyl ether, glycerol triglycidyl ether,trimethylolpropane triglycidyl ether, polyethylene glycol diglycidylether, polypropylene glycol diglycidyl ether, 1,13-tetradecadienedioxide, limonene dioxide, 1,2,7,8-diepoxyoctane, and1,2,5,6-diepoxycyclooctane.

Among these epoxy compounds, the aromatic epoxides and the alicyclicepoxides are preferable from the viewpoint of excellent curing speed,and the alicyclic epoxides are particularly preferable.

Examples of the vinyl ether compounds include di- or tri-vinyl ethercompounds such as ethylene glycol divinyl ether, diethylene glycoldivinyl ether, triethylene glycol divinyl ether, propylene glycoldivinyl ether, dipropylene glycol divinyl ether, butanediol divinylether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether,and trimethylolpropane trivinyl ether, and monovinyl ether compoundssuch as ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether,octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether,2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether,n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl vinylether-O-propylene carbonate, dodecyl vinyl ether, and diethylene glycolmonovinyl ether.

Detailed examples of monofunctional vinyl ethers and polyfunctionalvinyl ethers are given below.

Specific examples of monofunctional vinyl ethers include methyl vinylether, ethyl vinyl ether, propyl vinyl ether, n-butyl vinyl ether,t-butyl vinyl ether, 2-ethylhexyl vinyl ether, n-nonyl vinyl ether,lauryl vinyl ether, cyclohexyl vinyl ether, cyclohexylmethyl vinylether, 4-methylcyclohexylmethyl vinyl ether, benzyl vinyl ether,dicyclopentenyl vinyl ether, 2-dicyclopentenoxyethyl vinyl ether,methoxyethyl vinyl ether, ethoxyethyl vinyl ether, butoxyethyl vinylether, methoxyethoxyethyl vinyl ether, ethoxyethoxyethyl vinyl ether,methoxypolyethylene glycol vinyl ether, tetrahydrofurfuryl vinyl ether,2-hydroxyethyl vinyl ether, 2-hydroxypropyl vinyl ether, 4-hydroxybutylvinyl ether, 4-hydroxymethylcyclohexylmethyl vinyl ether, diethyleneglycol monovinyl ether, polyethylene glycol vinyl ether, chloroethylvinyl ether, chlorobutyl vinyl ether, chloroethoxyethyl vinyl ether,phenylethyl vinyl ether, and phenoxypolyethylene glycol vinyl ether.

Furthermore, examples of polyfunctional vinyl ethers include divinylethers such as ethylene glycol divinyl ether, diethylene glycol divinylether, polyethylene glycol divinyl ether, propylene glycol divinylether, butylene glycol divinyl ether, hexanediol divinyl ether,bisphenol A alkylene oxide divinyl ether, and bisphenol F alkylene oxidedivinyl ether; and polyfunctional vinyl ethers such as trimethylolethanetrivinyl ether, trimethylolpropane trivinyl ether, ditrimethylolpropanetetravinyl ether, glycerol trivinyl ether, pentaerythritol tetravinylether, dipentaerythritol pentavinyl ether, dipentaerythritol hexavinylether, an ethylene oxide adduct of trimethylolpropane trivinyl ether, apropylene oxide adduct of trimethylolpropane trivinyl ether, an ethyleneoxide adduct of ditrimethylolpropane tetravinyl ether, a propylene oxideadduct of ditrimethylolpropane tetravinyl ether, an ethylene oxideadduct of pentaerythritol tetravinyl ether, a propylene oxide adduct ofpentaerythritol tetravinyl ether, an ethylene oxide adduct ofdipentaerythritol hexavinyl ether, and a propylene oxide adduct ofdipentaerythritol hexavinyl ether.

As the vinyl ether compound, the di- or tri-vinyl ether compounds arepreferable from the viewpoint of curability, adhesion to a recordingmedium, surface hardness of the image formed, etc., and the divinylether compounds are particularly preferable.

The oxetane compound in the present invention means a compound having atleast one oxetane ring, and may be selected freely from known oxetanecompounds such as those described in JP-A-2001-220526, JP-A-2001-310937,and JP-A-2003-341217.

As the compound having an oxetane ring that can be used in the inkcomposition of the present invention, a compound having 1 to 4 oxetanerings in the structure is preferable. In accordance with use of such acompound, it becomes easy to maintain the viscosity of the inkcomposition in a range that gives good handling properties and,furthermore, the cured ink can be given high adhesion to the recordingmedium, which is preferable.

Examples of compounds having 1 to 2 oxetane rings in the moleculeinclude compounds represented by Formulae (1) to (3) below.

R^(a1) denotes a hydrogen atom, an alkyl group having 1 to 6 carbons, afluoroalkyl group having 1 to 6 carbons, an allyl group, an aryl group,a furyl group, or a thienyl group. When there are two R^(a1) in themolecule, they may be identical to or different from each other.

Examples of the alkyl group include a methyl group, an ethyl group, apropyl group, and a butyl group, and preferred examples of thefluoroalkyl group include those obtained by substituting any of thehydrogen atoms of the above alkyl groups with a fluorine atom.

R^(a2) denotes a hydrogen atom, an alkyl group having 1 to 6 carbons, analkenyl group having 2 to 6 carbons, a group having an aromatic ring, analkylcarbonyl group having 2 to 6 carbons, an alkoxycarbonyl grouphaving 2 to 6 carbons, or an N-alkylcarbamoyl group having 2 to 6carbons. Examples of the alkyl group include a methyl group, an ethylgroup, a propyl group, and a butyl group, examples of the alkenyl groupinclude a 1-propenyl group, a 2-propenyl group, a 2-methyl-1-propenylgroup, a 2-methyl-2-propenyl group, a 1-butenyl group, a 2-butenylgroup, and a 3-butenyl group, and examples of the group having anaromatic ring include a phenyl group, a benzyl group, a fluorobenzylgroup, a methoxybenzyl group, and a phenoxyethyl group. Examples of thealkylcarbonyl group include an ethylcarbonyl group, a propylcarbonylgroup, and a butylcarbonyl group, examples of the alkoxycarbonyl groupinclude an ethoxycarbonyl group, a propoxycarbonyl group, and abutoxycarbonyl group, and examples of the N-alkylcarbamoyl group includean ethylcarbamoyl group, a propylcarbamoyl group, a butylcarbamoylgroup, and a pentylcarbamoyl group. Furthermore, it is possible for R²to have a substituent, and the examples of the substituent include alkylgroup, having 1 to 6 carbons and fluorine atom

R^(a3) denotes a linear or branched alkylene group, a linear or branchedpoly(alkyleneoxy) group, a linear or branched unsaturated hydrocarbongroup, a carbonyl group, a carbonyl group-containing alkylene group, acarboxyl group-containing alkylene group, a carbamoyl group-containingalkylene group, or a group shown below. Examples of the alkylene groupinclude an ethylene group, a propylene group, and a butylene group, andexamples of the poly(alkyleneoxy) group include a poly(ethyleneoxy)group and a poly(propyleneoxy) group. Examples of the unsaturatedhydrocarbon group include a propenylene group, a methylpropenylenegroup, and a butenylene group.

When R^(a3) is the above-mentioned polyvalent group, R^(a4) denotes ahydrogen atom, an alkyl group having 1 to 4 carbons, an alkoxy grouphaving 1 to 4 carbons, a halogen atom, a nitro group, a cyano group, amercapto group, a lower alkylcarboxyl group, a carboxyl group, or acarbamoyl group.

R^(a5) denotes an oxygen atom, a sulfur atom, a methylene group, NH, SO,SO₂, C(CF₃)₂, or, C(CH₃)₂.

R^(a6) denotes an alkyl group having 1 to 4 carbons or an aryl group,and n is an integer of 0 to 2,000. R^(a7) denotes an alkyl group having1 to 4 carbons, an aryl group, or a monovalent group having thestructure below. In the formula, Ra8 denotes an alkyl group having 1 to4 carbons or an aryl group, and m is an integer of 0 to 100.

Examples of the compound represented by Formula (1) include3-ethyl-3-hydroxymethyloxetane (OXT-101: manufactured by Toagosei Co.,Ltd.), 3-ethyl-3-(2-ethylhexyloxymethyl)oxetane (OXT-212: manufacturedby Toagosei Co., Ltd.), and 3-ethyl-3-phenoxymethyloxetane (OXT-211:manufactured by Toagosei Co., Ltd.). Examples of the compoundrepresented by Formula (2) include1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene (OXT-121: ToagoseiCo., Ltd.). Examples of the compound represented by Formula (3) includebis(3-ethyl-3-oxetanylmethyl) ether (OXT-221: Toagosei Co., Ltd.).

Examples of the compound having 3 to 4 oxetane rings in the moleculeinclude compounds represented by Formula (4) below.

In Formula (4), R^(a1) denotes the same as in Formula (1) above.Furthermore, examples of R^(a9), which is a polyvalent linking group,include a branched alkylene group having 1 to 12 carbons such as a grouprepresented by A to C below, a branched poly(alkyleneoxy) group such asa group represented by D below, and a branched polysiloxane group suchas a group represented by E below. j is 3 or 4.

In the above A, R^(a10) denotes a methyl group, an ethyl group, or apropyl group. Furthermore, in the above D, p is an integer of 1 to 10.

Moreover, as another embodiment of the oxetane compound that can besuitably used in the present invention, a compound having an oxetanering on a side chain, represented by Formula (5) below, can be cited.

In Formula (5), R^(a1) and R^(a8) denote the same as in theabove-mentioned formulae. R^(a11) is an alkyl group having 1 to 4carbons such as a methyl group, an ethyl group, a propyl group, or abutyl group, or a trialkylsilyl group, and r is 1 to 4.

Such compounds having an oxetane ring are described in detail inparagraph Nos. (0021) to (0084) of JP-A-2003-341217 above, and thecompounds described here may be suitably used in the present invention.

The oxetane compounds described in JP-A-2004-91556 can be used in thepresent invention. The details are described in paragraph Nos. (0022) to(0058).

Among the oxetane compounds used in the present invention, from theviewpoint of ink composition viscosity and tackiness, it is preferableto use a compound having one oxetane ring.

The ink composition of the present invention may comprise only one typeof cationically polymerizable compound or two or more types thereof incombination, but from the viewpoint of suppressing effectively shrinkageduring ink curing, it is preferable to use a combination of a vinylether compound and at least one type of compound selected from theoxetane compounds and the epoxy compounds.

The content of the cationically polymerizable compound in the inkcomposition is suitably in the range of 10 to 95 wt % relative to thetotal solids content of the ink composition, preferably 30 to 90 wt %,and more preferably 50 to 85 wt %.

(c) Colorant

The colorant that can be used in the present invention is notparticularly limited, but a pigment and an oil-soluble dye that haveexcellent weather resistance and rich color reproduction are preferable,and it may be selected from any known colorant such as a soluble dye. Itis preferable that the colorant that can be suitably used in the inkcomposition or the inkjet recording ink composition of the presentinvention does not function as a polymerization inhibitor in apolymerization reaction, which is a curing reaction. This is because thesensitivity of the curing reaction by actinic radiation should not bedegraded.

(c-1) Pigment

The pigment that can be used in the present invention is notparticularly limited and, for example, organic and inorganic pigmentshaving the numbers below described in the Color Index may be used.

That is, as a red or magenta pigment, Pigment Red 3, 5, 19, 22, 31, 38,43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1,81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149,166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, or 257,Pigment Violet 3, 19, 23, 29, 30, 37, 50, or 88, and Pigment Orange 13,16, 20, or 36; as a blue or cyan pigment, Pigment Blue 1, 15, 15:1,15:2, 15:3, 15:4, 15:6, 16, 17-1, 22, 27, 28, 29, 36, or 60; as a greenpigment, Pigment Green 7, 26, 36, or 50; as a yellow pigment, PigmentYellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97,108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180,185, or 193; as a black pigment, Pigment Black 7, 28, or 26; as a whitepigment, Pigment White 6, 18, or 21, etc. may be used according to theintended application.

(c-2) Oil-Soluble Dye

The oil-soluble dye that can be used in the present invention isexplained below.

The oil-soluble dye that can be used in the present invention means adye that is substantially insoluble in water. Specifically, thesolubility in water at 25° C. (the mass of dye that can be dissolved in100 g of water) is no greater than 1 g, preferably no greater than 0.5g, and more preferably no greater than 0.1 g. Therefore, the oil-solubledye means a so-called water-insoluble pigment or an oil-soluble dye, andamong these the oil-soluble dye is preferable.

In the present invention, the oil-soluble dye may be used singly or in acombination of two or more types. Furthermore, another colorant such asa water-soluble dye, a disperse dye, or a pigment may be contained asnecessary in a range that does not interfere with the effects of thepresent invention.

Among the oil-soluble dyes that can be used in the present invention, asa yellow dye, any may be used. Examples thereof include aryl or heterylazo dyes having a coupling component such as a phenol, a naphthol, ananiline, a pyrazolone, a pyridone, or an open-chain active methylenecompound; azomethine dyes having a coupling component such as anopen-chain active methylene compound; methine dyes such as benzylidenedyes and monomethineoxonol dyes; quinone dyes such as naphthoquinonedyes and anthraquinone dyes; and other dye species such asquinophthalone dyes, nitro/nitroso dyes, acridine dyes, and acridinonedyes.

Among the above-mentioned oil-soluble dyes that can be used in thepresent invention, as a magenta dye, any may be used. Examples thereofinclude aryl or heteryl azo dyes having a coupling component such as aphenol, a naphthol, or an aniline; azomethine dyes having a couplingcomponent such as a pyrazolone or a pyrazolotriazole; methine dyes suchas arylidene dyes, styryl dyes, merocyanine dyes, and oxonol dyes;carbonium dyes such as diphenylmethane dyes, triphenylmethane dyes, andxanthene dyes; quinone dyes such as naphthoquinones, anthraquinones, oranthrapyridones; and condensed polycyclic dyes such as dioxazine dyes.

Among the oil-soluble dyes that can be used in the present invention, asa cyan dye, any may be used. Examples thereof include indoaniline dyes,indophenol dyes, and azomethine dyes having a coupling component such asa pyrrolotriazole; polymethine dyes such as cyanine dyes, oxonol dyes,and merocyanine dyes; carbonium dyes such as diphenylmethane dyes,triphenylmethane dyes, and xanthene dyes; phthalocyanine dyes;anthraquinone dyes; aryl or heteryl azo dyes having a coupling componentsuch as a phenol, a naphthol, or an aniline; and indigo/thioindigo dyes.

The above-mentioned dyes may be dyes that exhibit respective colors ofyellow, magenta, and cyan only after a part of the chromophoredissociates, and in that case the counter cation may be an inorganiccation such as an alkali metal or ammonium, may be an organic cationsuch as pyridinium or a quaternary ammonium salt, or may be a polymercation having the above cation as a partial structure.

Although not limited to the following, preferred specific examplesthereof include CI Solvent Black 3, 7, 27, 29, and 34; CI Solvent Yellow14, 16, 19, 29, 30, 56, 82, 93, and 162; CI Solvent Red 1, 3, 8, 18, 24,27, 43, 49, 51, 72, 73, 109, 122, 132, and 218; CI Solvent Violet 3; CISolvent Blue 2, 11, 25, 35, 38, 67, and 70; CI Solvent Green 3 and 7;and CI Solvent Orange 2. Particularly preferred examples thereof includeNubian Black PC-0850, Oil Black HBB, Oil Yellow 129, Oil Yellow 105, OilPink 312, Oil Red 5B, Oil Scarlet 308, Vali Fast Blue 2606, Oil Blue BOS(manufactured by Orient Chemical Industries, Ltd.), Aizen Spilon BlueGNH (manufactured by Hodogaya Chemical Co., Ltd.), Neopen Yellow 075,Neopen Magenta SE1378, Neopen Blue 808, Neopen Blue FF4012, and NeopenCyan FF4238 (manufactured by BASF).

In the present invention, a disperse dye may be used in a range thatenables it to be dissolved in a water-immiscible organic solvent.Specific preferred examples thereof include CI Disperse Yellow 5, 42,54, 64, 79, 82, 83, 93, 99, 100, 119, 122, 124, 126, 160, 184:1, 186,198, 199, 201, 204, 224, and 237; CI Disperse Orange 13, 29, 31:1, 33,49, 54, 55, 66, 73, 118, 119, and 163; CI Disperse Red 54, 60, 72, 73,86, 88, 91, 92, 93, 111, 126, 127, 134, 135, 143, 145, 152, 153, 154,159, 164, 167:1, 177, 181, 204, 206, 207, 221, 239, 240, 258, 277, 278,283, 311, 323, 343, 348, 356, and 362; CI Disperse Violet 33; CIDisperse Blue 56, 60, 73, 87, 113, 128, 143, 148, 154, 158, 165, 165:1,165:2, 176, 183, 185, 197, 198, 201, 214, 224, 225, 257, 266, 267, 287,354, 358, 365, and 368; and CI Disperse Green 6:1 and 9.

Particularly preferred examples of the oil-soluble dye include azo andazomethine dyes represented by Formulae (1) and (2) below. Dyesrepresented by Formula (2) below are known, in the photographic materialarea, as dyes that are generated from a coupler and a developing agentby oxidation.

In Formulae (1) and (2) above, R¹, R², R³ and R⁴ independently denote ahydrogen atom, a halogen atom, an aliphatic group, an aromatic group, aheterocyclic group, a cyano group, a hydroxyl group, a nitro group, anamino group, an alkylamino group, an alkoxy group, an aryloxy group, anamide group, an arylamino group, a ureido group, a sulfamoylamino group,an alkylthio group, an arylthio group, an alkoxycarbonylamino group, asulfonamide group, a carbamoyl group, a sulfamoyl group, a sulfonylgroup, an alkoxycarbonyl group, a heterocyclooxy group, an azo group, anacyloxy group, a carbamoyloxy group, a silyloxy group, anaryloxycarbonyl group, an aryloxycarbonylamino group, an imide group, aheterocyclothio group, a sulfinyl group, a phosphoryl group, an acylgroup, a carboxyl group, or a sulfo group.

In Formulae (1) and (2) above, in particular, R² is preferably, amongthe above-mentioned substituents, a hydrogen atom, a halogen atom, analiphatic group, an alkoxy group, an aryloxy group, an amide group, aureido group, a sulfamoylamino group, an alkoxycarbonylamino, or asulfonamide group.

In the present specification, the aliphatic group denotes an alkylgroup, a substituted alkyl group, an alkenyl group, a substitutedalkenyl group, an alkynyl group, a substituted alkynyl group, an aralkylgroup, or a substituted aralkyl group. The aliphatic group may have abranch or form a ring. The number of carbon atoms of the aliphatic groupis preferably 1 to 20, and more preferably 1 to 18. The aryl moiety ofthe aralkyl group and the substituted aralkyl group is preferably phenylor naphthyl, and particularly preferably phenyl. Examples of thesubstituents of the alkyl moieties of the substituted alkyl group, thesubstituted alkenyl group, the substituted alkynyl group, and thesubstituted aralkyl group include the substituents cited for explanationof R¹ to R⁴. Examples of the substituents of the aryl moiety of thesubstituted aralkyl group are the same as those of the substituent ofthe substituted aryl group below.

In the present specification, the aromatic group means an aryl group anda substituted aryl group. The aryl group is preferably phenyl ornaphthyl, and particularly preferably phenyl. The aryl moiety of thesubstituted aryl group is the same as that of the above-mentioned arylgroup. Examples of the substituent of the substituted aryl group includesubstituents cited for explanation of R¹ to R⁴.

In Formulae (1) and (2) above, A denotes —NR⁵R⁶ or a hydroxyl group, andR⁵ and R⁶ independently denote a hydrogen atom, an aliphatic group, anaromatic group, or a heterocyclic group. A is preferably —NR⁵R . R⁵ andR⁶ may be bonded together to form a ring. R⁵ and R⁶ preferably eachdenote a hydrogen atom, an alkyl group, a substituted alkyl group, anaryl group, or a substituted aryl group, and most preferably a hydrogenatom, an alkyl group having 1 to 18 carbon atoms, or a substituted alkylgroup having 1 to 18 carbon atoms.

In Formula (2) above, B¹ denotes ═C(R³)— or ═N—, and B² denotes —C(R⁴)═or —N═. It is preferable that B¹ and B² are not —N═ at the same time,and it is more preferable that B¹ is ═C(R³)— and B² is —C(R⁴)═. Any ofR¹ and R⁵, R³ and R⁶, and R¹ and R² may be bonded together to form anaromatic ring or a hetero ring.

In Formula (1) above, Y denotes an unsaturated heterocyclic group. Y ispreferably a five-membered or six-membered unsaturated hetero ring. Thehetero ring may be condensed with an aliphatic ring, an aromatic ring,or another hetero ring. Examples of the hetero atom of the hetero ringinclude N, O, and S.

Preferred examples of the above-mentioned unsaturated hetero ringinclude a pyrazole ring, an imidazole ring, a thiazole ring, anisothiazole ring, a thiadiazole ring, a thiophene ring, a benzothiazolering, a benzoxazole ring, a benzoisothiazole ring, a pyrimidine ring, apyridine ring, and a quinoline ring. It is also possible for theunsaturated heterocyclic group to have a substituent cited above as R¹to R⁴.

In Formula (2) above, X denotes a color photographic coupler residue.Preferred examples of the color photographic coupler residue are asfollows.

Yellow couplers: U.S. Pat. Nos. 3,933,501, 4,022,620, 4,326,024, and4,401,752, couplers represented by Formulae (I) and (II) in U.S. Pat.No.4,248,961, JP-B-58-10739, GB Pat. Nos. 1,425,020 and 1,476,760, U.S.Pat. Nos. 3,973,968, 4,314,023, and 4,511,649, and EP Pat. Nos. 249,473Aand 502,424A; couplers represented by Formulae (1) and (2) in EP Pat.No. 513,496A (in particular, Y-28 on page 18); couplers represented byFormula (I) of claim 1 in EP Pat. No. 568,037A; couplers represented byFormula (I) of lines 45 to 55 in Column 1 in U.S. Pat. No. 5,066,576;couplers represented by Formula (I) in Paragraph 0008 in JP-A-4-274425;couplers of claim 1 on page 40 in EP Pat. No. 498,381A1 (in particular,D-35 on page 18); couplers represented by Formula (Y) on page 4 in EPPat. No. 447,969A1 (in particular, Y-1 (page 17) and Y-54 (page 41));and couplers represented by Formulae (II) to (IV) on lines 36 to 58 ofColumn 7 in U.S. Pat. No. 4,476,219 (in particular, 11-17 and 19 (Column17), and 11-24 (Column 1 9)).

Magenta couplers: U.S. Pat. Nos. 4,310,619 and 4,351,897, EP Pat. No.73,636, U.S. Pat. Nos. 3,061,432 and 3,725,067, Research Disclosure No.24220 (June, 1984) and No. 24230 (June, 1984), JP-A-60-33552,JP-A-60-43659, JP-A-61-72238, JP-A-60-35730, JP-A-55-118034,JP-A-60-185951, U.S. Pat. Nos. 4,500,630, 4,540,654, and 4,556,630,WO88/04795, JP-A-3-39737 (L-57 (page 11, lower right), L-68 (page 12,lower right), L-77 (page 13, lower right)), EP Pat. No. 456,257 [A-4]-63(p. 134), [A-4]-73, -75 (p. 139), EP Pat. No. 486,965 M-4, -6 (p. 26),M-7 (p. 27), EP Pat. No. 571,959A M-45 (p. 19), JP-A-5-204106 M-1 (p.6), and JP-A-4-362631 paragraph No. 0237, M-22, and U.S. Pat. Nos.3,061,432 and 3,725,067.

Furthermore, couplers described in JP-A-62-215272 (p. 91), JP-A-2-33144(p. 3 and 30), EP 355,660A (p. 4, 5, 45 and 47) are also useful.

Cyan coupler: U.S. Pat. Nos. 4,052,212, 4,146,396, 4,228,233, and4,296,200, EP Pat. No. 73,636, JP-A-4-204843, CX-1, 3, 4, 5, 11, 12, 14,15 (pp. 14 to 16); JP-A-4-43345 C-7, 10 (p. 35), 34, 35 (p. 37), (I-1),(I-17) (pp. 42 to 43); and couplers represented by Formula (la) or (lb)of claim 1 in JP-A-6-67385.

Among the oil-soluble dyes represented by Formula (1) above, the magentadyes particularly preferably used are dyes represented by Formula (3)below.

In Formula (3) above, Z¹ denotes an electron-attracting group having aHammett substituent constant σp value of equal to or greater than 0.20.Z¹ is preferably an electron-attracting group having a σp value of atleast 0.30 but no greater than 1.0. Preferred specific examples of thesubstituent include electron-attracting substituents that are describedlater, and among them an acyl group having 2 to 12 carbons, an alkyloxycarbonyl group having 2 to 12 carbons, a nitro group, a cyano group, analkylsulfonyl group having 1 to 12 carbons, an arylsulfonyl group having6 to 18 carbons, a carbamoyl group having 1 to 12 carbons, and ahaloalkyl group having 1 to 12 carbons are preferable. A cyano group, analkylsulfonyl group having 1 to 12 carbons, and an arylsulfonyl grouphaving 6 to 18 carbons are particularly preferable, and a cyano group ismost preferable.

In Formula (3) above, Z² denotes a hydrogen atom, an aliphatic group, oran aromatic group.

In Formula (3) above, R¹ to R⁶ are the same as the corresponding ones ofFormula (1) above.

In Formula (3) above, Q denotes a hydrogen atom, an aliphatic group, anaromatic group, or a heterocyclic group. Among them, Q is preferably agroup formed from a group of non-metal atoms necessary to form a 5- to8-membered ring. Among them an aromatic group and a heterocyclic groupare particularly preferable. The 5- to 8-membered ring may besubstituted, may be a saturated ring, or may have an unsaturated bond.Preferred examples of the non-metal atom include a nitrogen atom, anoxygen atom, a sulfur atom, and a carbon atom. Specific examples of suchring structures include a benzene ring, a cyclopentane ring, acyclohexane ring, a cycloheptane ring, a cyclooctane ring, a cyclohexenering, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazinering, a triazine ring, an imidazole ring, a benzoimidazole ring, anoxazole ring, a benzoxazole ring, an oxane ring, a sulfolane ring, and athiane ring, and in a case where these rings have a further substituent,examples of the substituent include groups cited as examples ofsubstituents R¹ to R⁴ in Formula (1) above.

Preferred structures of the compounds represented by Formula (3) aboveare described in JP-A-2001-335714.

Among the dyes represented by Formula (2) above, the magenta dyeparticularly preferably employs a dye represented by Formula (4) below.

In Formula (4) above, G denotes a hydrogen atom, an aliphatic group, anaromatic group, a heterocyclic group, a cyano group, an alkoxy group, anaryloxy group, an alkylthio group, an arylthio group, an ester group, anamino group, a carbamoyl group, a sulfonyl group, a sulfamoyl group, aureido group, a urethane group, an acyl group, an amide group, or asulfonamide group.

In Formula (4) above, R¹, R², A, B¹, and B² are the same as thecorresponding ones of Formula (2) above, and preferred ranges are alsothe same.

In Formula (4) above, L denotes an atomic group forming a five-memberedor six-membered nitrogen-containing hetero ring, which may besubstituted with at least one of an aliphatic group, an aromatic group,a heterocyclic group, a cyano group, an alkoxy group, an aryloxy group,an alkylthio group, an arylthio group, an ester group, an amino group, acarbamoyl group, a sulfonyl group, a sulfamoyl group, a ureido group, aurethane group, an acyl group, an amide group, and a sulfonamide group,and this hetero ring may further form a condensed ring with anotherring.

With regard to compounds represented by Formula (4) above, A ispreferably —NR⁵R⁶, and L preferably forms a five-memberednitrogen-containing hetero ring; examples of the five-memberednitrogen-containing hetero ring include an imidazole ring, a triazolering, and a tetrazole ring.

Among the dyes represented by Formula (1) and Formula (2) above,compound examples (M0, M-1 to 6, a-21 to 25) for a magenta dye are shownbelow, but these are only for explaining the present invention indetail, and the present invention should not be construed as beinglimited thereto.

In the present invention, M0, M-4, M-6, or a-21 may be used, and M-4,M-6, and a-21 are particularly preferable.

TABLE 1

R₁ a-21

a-22

a-23

a-24

a-25

R₂ a-21

a-22

a-23

a-24

a-25

R₃ R₄ a-21

a-22

a-23

a-24

a-25

C₈H₁₇(t)

Other compound examples of the colorant that can be used in the presentinvention include those described in JP-A-2001-240763, 2001-181549, andJP-A-2001-335714, but the present invention should not be construed asbeing limited thereto.

The compound represented by Formula (3) above may be synthesized byreference to a method described in, for example, JP-A-2001-335714 orJP-A-55-161856. The compound represented by Formula (4) above may besynthesized by reference to a method described in, for example,JP-A-4-126772, JP-B-7-94180, or JP-A-2001-240763.

Among the dyes represented by Formula (2) above, as a cyan dye apyrrolotriazole azomethine dye represented by Formula (5) below isparticularly preferably used.

In Formula (5) above, A, R¹, R², B¹, and B² are the same as thecorresponding ones of Formula (2) above, and preferred ranges thereofare also the same.

In Formula (5) above, Z³ and Z⁴ are independently the same as G inFormula (4) above. Z³ and Z⁴ may be bonded together to form a ringstructure. One in which Z³ is an electron-attracting group having aHammett substituent constant up value of equal to or greater than 0.30exhibits a sharp absorption and is more preferable. Moreover, one inwhich Z³ is an electron-attracting group having a Hammett substituentconstant up value of equal to or greater than 0.45 is more preferable,and an electron-attracting group having a Hammett substituent constantσp value of equal to or greater than 0.60 is most preferable.Furthermore, one in which the sum of the Hammett substituent constant upvalues of Z³ and Z⁴ is equal to or greater than 0.70 exhibits excellenthue of a cyancolor, and is more preferable.

In Formula (5) above, M is an atomic group forming a 1,2,4-triazole ringthat is condensed with the 5-membered ring of Formula (5) above; eitherone of the two atoms B³ and B⁴ at the sites of condensation with the5-membered ring is a nitrogen atom, and the other is a carbon atom.

The compound represented by Formula (5) above is preferably used as acyan dye, but it may be used as a magenta dye by changing a substituent.

The Hammett substituent constant up value used in the presentspecification is now- explained. The Hammett rule is an empirical ruleproposed by L. P. Hammett in 1935 in order to quantitatively deal withthe influence of a substituent on a reaction or equilibrium of a benzenederivative, and the validity thereof is currently widely recognized. Aσp value and a σm value are required for the substituent constant in theHammett rule, and details of these values can be referred to in manygeneral books, for example, ‘Lange's Handbook of Chemistry’, Ed. by J.A. Dean, 12th edition, 1979 (Mc Graw-Hill) or ‘Kagakuno Ryouiki’(Journal of Japanese Chemistry), special issue, 122, pp. 96 to 103, 1979(Nankodo Co., Ltd.). In the present invention, the substituents arelimited or explained using the Hammett substituent constant σp, but thisdoes not mean that they are limited to substituents whose values aredescribed in published references, and a substituent whose value is notpublished in the references but is included in the range if it ismeasured in accordance with the Hammett rule is of course included.Among Formulae (1) to (5) above, those that are not benzene derivativesare also included, but the σp value is used as a scale showing theelectronic effect of the substituent, irrespective of the position ofsubstitution. The σp value in the present invention is used with theabove-mentioned meaning.

Examples of electron-attracting groups having a Hammett substituentconstant σp value of equal to or greater than 0.60 include a cyanogroup, a nitro group, an alkylsulfonyl group (e.g. a methanesulfonylgroup), and an arylsulfonyl group (e.g. a benzenesulfonyl group).Examples of electron-attracting groups having a Hammett σp value ofequal to or greater than 0.45 include, in addition to the above, an acylgroup (e.g. an acetyl group), an alkoxycarbonyl group (e.g. adodecyloxycarbonyl group), an aryloxycarbonyl group (e.g.m-chlorophenoxycarbonyl), an alkylsulfinyl group (e.g.n-propylsulfinyl), an arylsulfinyl group (e.g. phenylsulfinyl), asulfamoyl group (e.g. N-ethylsulfamoyl, N,N-dimethylsulfamoyl), and ahaloalkyl group (e.g. trifluoromethyl).

Examples of electron-attracting groups having a Hammett substituentconstant ap value of equal to or greater than 0.30 include, in additionto the above, an acyloxy group (e.g. acetoxy), a carbamoyl group (e.g.N-ethylcarbamoyl, N,N-dibutylcarbamoyl), a haloalkoxy group (e.g.trifluoromethyloxy), a haloaryloxy group (e.g. pentafluorophenyloxy), asulfonyloxy group (e.g. a methylsulfonyloxy group), a haloalkylthiogroup (e.g. difluoromethylthio), an aryl group substituted with two ormore electron-attracting groups having a σp value of equal to or greaterthan 0.15 (e.g. 2,4-dinitrophenyl, pentachlorophenyl), and a hetero ring(e.g. 2-benzooxazolyl, 2-benzothiazolyl, 1-phenyl-2-benzimidazolyl).Specific examples of electron-attracting groups having a σp value ofequal to or greater than 0.20 include, in addition to the above, ahalogen atom.

Furthermore, in the present invention, an oil-soluble dye represented byFormula (A-I) below can be used preferably.

In Formula (A-I): X₁, X₂, X₃, and X₄ independently denote a groupselected from —SO-Z, —SO₂-Z, —SO₂NR₁R₂, —CONR₁R₂, —CO₂R₁, and a sulfogroup. Here, Z denotes a substituted or unsubstituted alkyl group, asubstituted or unsubstituted cycloalkyl group, a substituted orunsubstituted alkenyl group, a substituted or unsubstituted aralkylgroup, a substituted or unsubstituted aryl group, or a substituted orunsubstituted heterocyclic group. R₁ and R₂ independently denote ahydrogen atom, a substituted or unsubstituted alkyl group, a substitutedor unsubstituted cycloalkyl group, a substituted or unsubstitutedalkenyl group, a substituted or unsubstituted aralkyl group, asubstituted or unsubstituted aryl group, or a substituted orunsubstituted heterocyclic group, provided that R₁ and R₂ are not bothhydrogen atoms. M denotes a hydrogen atom, a metal element, a metaloxide, a metal hydroxide, or a metal halide. Y₁, Y₂, Y₃, and Y₄independently denote a hydrogen atom or a monovalent substituent. a₁ toa₄ and b₁ to b₄ denote the numbers of X₁ to X₄ and Y₁ to Y₄, andindependently denote an integer of 0 to 4, provided that the sum totalof a₁ to a₄ is equal to or greater than 2.

Among the oil-soluble dyes represented by Formula (A-I) above, anoil-soluble dye represented by Formula (A-II) below may particularlypreferably be used.

In Formula (A-II): X₁₁ to X₁₄, Y₁₁ to Y₁₈, and M are the same as X₁ toX₄, Y₁ to Y₄, and M in Formula (A-I) respectively. a₁₁ to a₁₄independently denote an integer of 1 or 2.

As a specific example of Formula (A-II) above, a compound example(AII-17) is cited, but this is for explaining the present invention indetail, and the present invention should not be construed as beinglimited thereto.

Compound No. M X a AII-17 Cu

1 AII-18 Cu

1 AII-19 Cu

1 AII-20 Cu

1 AII-21 Cu

1 AII-22 Cu

1 AII-23 Cu

1

In the present invention, it is preferable to use an oil-soluble dyehaving an oxidation potential that is more noble than 1.0 V (SCE). Themore noble the oxidation potential, the more preferable it is; it ismore preferable to use one having an oxidation potential more noble than1.1 V (SCE), and it is most preferable to use one having an oxidationpotential more noble than 1.2 V (SCE).

The oxidation potential value (Eox) can be easily measured by oneskilled in the art and a method therefor is described in, for example,P. Delahay, ‘New Instrumental Methods in Electrochemistry’, IntersciencePublishers (1954), A. J. Bard et al., ‘Electrochemical Methods’, JohnWiley & Sons (1980), and Akira Fujishima et al., ‘DenkikagakuSokuteihou’ (Electrochemical Measurement Methods), Gihodo Shuppan Sha(1984).

More specifically, a test sample is dissolved to give a concentration of1×10⁻⁴ to 1×10⁻⁶ mol/L in a solvent such as dimethylformamide oracetonitrile containing a supporting electrolyte such as sodiumperchlorate or tetrapropylammonium perchlorate, an oxidation wave whensweeping toward the oxidation side (noble side) using carbon (GC) as aworking electrode and a rotating platinum electrode as the counterelectrode using cyclic voltammetry or direct current polarographicequipment is approximated to a straight line, and the oxidationpotential of the midpoint of a line segment formed between anintersection point of the straight line and a residual current/potentialstraight line and an intersection point of the straight line and asaturated current straight line (or an intersection point with astraight line parallel to the ordinate passing through the potentialpeak value) is measured as a value relative to the SCE (saturatedcalomel electrode). This value sometimes deviates by on the order oftens of millivolts due to the effect of a liquid junction potential, theliquid resistance of the sample solution, or the like, but thereproducibility of the potential can be guaranteed by adding a standardsample (for example, hydroquinone). The support electrolyte and solventused may be selected appropriately according to the oxidation potentialand the solubility of the test sample. The support electrolyte andsolvent that can be used here may be referred to in Akira Fujishima etal., ‘Denkikagaku Sokuteihou’ (Electrochemical Measurement Methods),Gihodo Shuppan Sha (1984), pp.101 to 118.

In the concentration range of the above-mentioned measurement solventand a phthalocyanine compound sample, the oxidation potential of adisassociated state is measured.

The value of Eox represents the ease of electron transfer from a sampleto an electrode; the larger the value (the more noble the oxidationpotential), the more difficult it is for electrons to transfer from thesample to the electrode, in other words, the more difficult it is tooxidize.

If a dye having a low oxidation potential is used, polymerization isgreatly inhibited by the dye, and the curability is degraded. When a dyehaving a noble oxidation potential is used, there is hardly anyinhibition of polymerization.

The colorant that can be used in the present invention is preferablyadded to the ink composition or the inkjet recording ink composition ofthe present invention and then dispersed in the ink to an appropriatedegree. For dispersion of the colorant, for example, a dispersingmachine such as a ball mill, a sand mill, an attritor, a roll mill, anagitator, a Henschel mixer, a colloidal mill, an ultrasonic homogenizer,a pearl mill, a wet type jet mill, or a paint shaker may be used.

When carrying out dispersion of the colorant, a dispersant may be added.The type of dispersant is not particularly limited, but it is preferableto use a polymeric dispersant, and examples of the polymeric dispersantinclude the Solsperse series manufactured by Avecia. Furthermore, as adispersion adjuvant, it is also possible to use a synergist, accordingto the various types of pigment. In the present invention, thedispersant and dispersion adjuvant are preferably added at 1 to 50 partsby weight relative to 100 parts by weight of the pigment.

The colorant may be added directly to the ink composition of the presentinvention, but in order to improve dispersibility it may be added inadvance to a solvent or a dispersing medium such as a polymerizablecompound used in the present invention. In the present invention, inorder to avoid the problem of the solvent resistance being degraded whenthe solvent remains in the cured image and the VOC (Volatile OrganicCompound) problem of the residual solvent, it is preferable to add thecolorant to a polymerizable compound. As a polymerizable compound used,it is preferable in terms of dispersion suitability to select a monomerhaving the lowest viscosity.

In the present invention, it is preferable for the average particle sizeof the colorant to be in the range of 0.005 to 0.5 μm, more preferably0.01 to 0.45 μm, and yet more preferably, 0.015 to 0.3 μm. Furthermore,it is preferable for the maximum particle size of the colorant to be 0.3to 10 μm, and more preferably 0.3 to 3 μm. It is preferable, in order tomake the maximum particle size of the pigment particles be in theabove-mentioned range, that the colorant, the dispersant, and thedispersing medium are selected, and dispersion conditions and filtrationconditions are set. By such control of particle size, clogging of a headnozzle can be suppressed, and the ink storage stability, the inktransparency, and the curing sensitivity can be maintained, which ispreferable.

(d) Sensitizing Dye

The ink composition of the present invention may contain (d) asensitizing dye in order to promote decomposition of the above-mentionedpolymerization initiator by absorbing specific actinic radiation, inparticular when used for inkjet recording. The sensitizing dye absorbsspecific actinic radiation and attains an electronically excited state.The sensitizing dye in the electronically excited state causes actionssuch as electron transfer, energy transfer, or heat generation uponcontact with the polymerization initiator. This causes thepolymerization initiator to undergo a chemical change and decompose,thus forming a radical, an acid, or a base.

Preferred examples of the sensitizing dye include those in thecategories of compounds below and have an adsorption wavelength in theregion of 350 nm to 450 nm.

Examples thereof include polynuclear aromatic compounds (e.g. pyrene,perylene, triphenylene), xanthenes (e.g. fluorescein, eosin,erythrosine, rhodamine B, rose bengal), cyanines (e.g. thiacarbocyanine,oxacarbocyanine), merocyanines (e.g. merocyanine, carbomerocyanine),thiazines (e.g. thionine, methylene blue, toluidine blue), acridines(e.g. acridine orange, chloroflavine, acriflavine), anthraquinones (e.g.anthraquinone), squaryliums (e.g. squarylium), and coumarins (e.g.7-diethylamino-4-methylcoumarin).

Preferred examples of the sensitizing dye that can be used in thepresent invention include compounds represented by Formulae (vi) to (x)below.

In Formula (vi), A¹ denotes a sulfur atom or NR⁵⁰, and R⁵⁰ denotes analkyl group or an aryl group. L¹ denotes a non-metallic atomic groupforming a basic nucleus of a dye in cooperation with the adjacent A¹ andadjacent carbon atom. R⁵¹ and R⁵² independently denote a hydrogen atomor a monovalent non-metallic atomic group, and R⁵¹ and R⁵² may be bondedtogether to form an acidic nucleus of a dye. W denotes an oxygen atom ora sulfur atom.

In Formula (vii), Ar¹ and Ar² independently denote an aryl group and areconnected to each other via bonding with L². Here, L² denotes —O— or—S—. W has the same meaning as that shown in Formula (vi).

In Formula (viii), A² denotes a sulfur atom or NR⁵⁹, and R⁵⁹ denotes analkyl group or an aryl group. L³ denotes a non-metallic atomic groupforming a basic nucleus of a dye in cooperation with the adjacent A² andcarbon atom. R⁵³, R⁵⁴, R⁵⁵, R⁵⁶, R⁵⁷, and R⁵⁸ independently denote amonovalent non-metallic atomic group.

In Formula (ix), A³ and A⁴ independently denote —S—, —NR⁶²—, or —NR⁶³—,and R⁶² and R⁶³ independently denote a substituted or unsubstitutedalkyl group or a substituted or unsubstituted aryl group. L⁴ and L⁵independently denote a non-metallic atomic group forming a basic nucleusof a dye in cooperation with the adjacent A³, A⁴, and adjacent carbonatom. R⁶⁰ and R⁶¹ independently denote a hydrogen atom or a monovalentnon-metallic atomic group. R⁶⁰ and R⁶¹ may be bonded to each other toform an aliphatic or aromatic ring.

In Formula (x), R⁶⁶ denotes an aromatic ring or a hetero ring, which mayhave a substituent. A⁵ denotes an oxygen atom, a sulfur atom, or —NR⁶⁷—.R⁶⁴, R⁶⁵, and R⁶⁷ independently denote a hydrogen atom or a monovalentnon-metallic atomic group. R⁶⁷ and R⁶⁴, and R⁶⁵ and R⁶⁷ may be bonded toeach other to form an aliphatic or aromatic ring.

In Formula (XI), A₂ denotes a sulfur atom or NR⁵⁹, L⁴ denotes anon-metallic atomic group forming a basic nucleus of a dye incooperation with the adjacent A₂ and carbon atom, R⁵³, R⁵⁴, R⁵⁵, R⁵⁶,R⁵⁷, and R⁵⁸ independently denote a monovalent non-metallic atomicgroup, and R⁵⁹ denotes an alkyl group or an aryl group.

In Formula (XII), A³ and A⁴ independently denote —S—, —NR⁶²—, or —NR⁶³—,R⁶² and R⁶³ independently denote a substituted or unsubstituted alkylgroup, or a substituted or unsubstituted aryl group, L⁵ and L⁶independently denote a non-metallic atomic group forming a basic nucleusof a dye in cooperation with the adjacent A³ and A⁴ and adjacent carbonatom, and R⁶⁰ and R⁶¹ independently denote a hydrogen atom or amonovalent non-metallic atomic group, or are bonded to each other toform an aliphatic or aromatic ring.

In Formula (XIII), R⁶⁶ denotes an aromatic ring or a hetero ring, whichmay have a substituent, and A⁵ denotes an oxygen atom, a sulfur atom, or—NR⁶⁷—. R⁶⁴, R⁶⁵, and R⁶⁷ independently denote a hydrogen atom or amonovalent non-metallic atomic group, and R⁶⁷ and R⁶⁴, and R⁶⁵ and R⁶⁷may be bonded to each other to form an aliphatic or aromatic ring.

In Formula (XIV), R₆₈ and R₆₉ independently denote a hydrogen atom or amonovalent non-metallic atomic group, R₇₀ and R₇₁ independently denote ahydrogen atom or a monovalent non-metallic atomic group, and n denotesan integer of 0 to 4. When n is 2 or greater, R₇₀ and R₇₁ may be bondedto each other to form an aliphatic or aromatic ring.

Preferred examples of the compounds represented by Formulae (vi) to(XIV) include (C-1) to (C-26) below, but are not limited thereto. Thestructural formula of the compounds below is illustrated by a simplifiedstructural formula, and solid lines, etc. which are not specificallydesignated by an element or a substituent denote a hydrocarbon group.

From the viewpoint of the coloring properties of the ink, the content ofthe sensitizing dye (d) in the ink composition of the present inventionis preferably 0.01 to 20 wt % relative to the entire solids content ofthe ink composition, more preferably 0.1 to 15 wt %, and yet morepreferably 0.5 to 10 wt %.

The sensitizing dye (d) may be used singly or in a combination of two ormore types.

Furthermore, from the viewpoint of improvement of decompositionefficiency of the specific onium salt and transparency to irradiatinglight, the ratio of the specific onium salt (a) to the sensitizing dye(d) contained in the ink composition is preferably a/d=100 to 0.05 as aratio by weight, more preferably a/d=50 to 0.1, and yet more preferablya/d=10 to 0.5.

(e) Cosensitizer

The ink composition of the present invention preferably comprises acosensitizer. In the present invention, the cosensitizer has thefunction of further improving the sensitivity of the sensitizing dye toactinic radiation or the function of suppressing inhibition by oxygen ofpolymerization of a polymerizable compound, etc.

Examples of such a cosensitizer include amines such as compoundsdescribed in M. R. Sander et al., ‘Journal of Polymer Society’, Vol. 10,p. 3173 (1972), JP-B-44-20189, JP-A-51 -82102, JP-A-52-134692,JP-A-59-138205, JP-A-60-84305, JP-A-62-18537, JP-A-64-33104, andResearch Disclosure No. 33825, and specific examples thereof includetriethanolamine, ethyl p-dimethylaminobenzoate, p-formyldimethylaniline,and p-methylthiodimethylaniline.

Other examples of the cosensitizer include thiols and sulfides such asthiol compounds described in JP-A-53-702, JP-B-55-500806, andJP-A-5-142772, and disulfide compounds of JP-A-56-75643, and specificexamples thereof include 2-mercaptobenzothiazole, 2-mercaptobenzoxazole,2-mercaptobenzimidazole, 2-mercapto-4(3H)-quinazoline, andβ-mercaptonaphthalene.

Yet other examples of the cosensitizer include amino acid compounds(e.g. N-phenylglycine, etc.), organometallic compounds described inJP-B-48-42965 (e.g. tributyltin acetate, etc.), hydrogen-donatingcompounds described in JP-B-55-34414, sulfur compounds described inJP-A-6-308727 (e.g. trithiane, etc.), phosphorus compounds described inJP-A-6-250387 (diethylphosphite, etc.), and Si—H or Ge—H compoundsdescribed in JP-A-8-54735.

(f) Other Polymerizable Compound

The ink composition of the present invention may employ the cationicallypolymerizable compound (b) and another polymerizable compound incombination. Examples of polymerizable compounds that can be used incombination in the present invention include radically polymerizablecompounds. As the radically polymerizable compounds, photocurablematerials employing a photopolymerizable composition described inJP-A-7-159983, JP-B-7-31399, JP-A-8-224982, JP-A-10-863, JP-A-9-80675,etc. are known (JP-B denotes a Japanese examined patent applicationpublication).

The radically polymerizable compounds that can be used in the presentinvention are compounds having at least one radically polymerizablecarbon-carbon double bond (ethylenically unsaturated bond), may be anycompound as long as it has one radically polymerizable ethylenicallyunsaturated bond in the molecule, and include chemical configurationssuch as monomer, oligomer, and polymer. The radically polymerizablecompounds may be used singly or may be used in a combination of two ormore types at any ratio in order to improve desired properties.

Examples of the polymerizable compound having a radically polymerizableethylenically unsaturated bond include unsaturated carboxylic acids suchas acrylic acid, methacrylic acid, itaconic acid, crotonic acid,isocrotonic acid, and maleic acid, and salts thereof, anhydrides havingan ethylenically unsaturated group, acrylonitrile, styrene, and varioustypes of radically polymerizable compounds such as unsaturatedpolyesters, unsaturated polyethers, unsaturated polyamides, andunsaturated urethanes.

Specific examples thereof include acrylic acid derivatives such as2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, butoxyethyl acrylate,carbitol acrylate, cyclohexyl acrylate, tetrahydrofurfuryl acrylate,benzyl acrylate, diacetone acrylamide, and epoxyacrylate; methacrylicderivatives such as methyl methacrylate, n-butyl methacrylate,2-ethylhexyl methacrylate, lauryl methacrylate, allyl methacrylate,glycidyl methacrylate, benzyl methacrylate, and dimethylaminomethylmethacrylate; and allyl compound derivatives such as allyl glycidylether. More specifically, commercial products, radically polymerizableor crosslinking monomers, oligomers, and polymers known in the art suchas those described in ‘Kakyozai Handobukku’ (Crosslinking AgentHandbook), Ed. S. Yamashita (Taiseisha, 1981); ‘UV.EB Koka Handobukku’(UV.EB Curing Handbook (Starting Materials) Ed. K. Kato (KobunshiKankoukai, 1985); ‘UV.EB Koka Gijutsu no Oyo to Shijyo’ (Application andMarket of UV.EB Curing Technology', p. 79, Ed. Rad Tech (CMC, 1989); andE. Takiyama ‘Poriesuteru Jushi Handobukku’ (Polyester Resin Handbook),(The Nikkan Kogyo Shimbun Ltd., 1988) can be used.

It is preferable to use the radically polymerizable compound and thecationically polymerizable compound in combination since, due to highsensitivity characteristic of radical polymerization and low volumeshrinkage characteristic of cationic polymerization, a printed materialor a lithographic printing plate having both high sensitivity andadhesion is obtained.

(g) Another Polymerization Initiator

The ink composition of the present invention may employ (a) the specificonium salt and (g) another polymerization initiator in combination. Asthe polymerization initiator employed in combination, known radicalpolymerization initiators can be cited. Said other polymerizationinitiators may be used singly or in a combination of two or more types.

The radical polymerization initiator that can be used in the inkcomposition of the present invention is a compound that forms apolymerization initiating species by absorbing external energy. Theexternal energy used for initiating polymerization can be broadlydivided into heat and actinic radiation, and a thermal polymerizationinitiator and a photopolymerization initiator are used respectively.Examples of the actinic radiation include γ rays, β rays, an electronbeam, UV rays, visible light, and IR rays. The polymerization initiatorthat can be used in the present invention is preferably aradiation-sensitive radical polymerization initiator that is sensitiveto actinic radiation, that is, a so-called radical photopolymerizationinitiator.

Examples of the radical polymerization initiator that can be used incombination in the present invention include (a) aromatic ketones, (b)organic peroxides, (c) thio compounds, (d) hexaarylbiimidazolecompounds, (e) ketoxime ester compounds, (f) borate compounds, (g)azinium compounds, (h) metallocene compounds, (i) active estercompounds, (j) compounds having a carbon-halogen bond, and (k)alkylamine compounds. These radical polymerization initiators may beused singly or in a combination of the above-mentioned compounds (a) to(k).

(h) Other Components

The ink composition of the present invention may comprise othercomponents as necessary. Examples of the other components include basiccompounds, polymerization inhibitors, solvents, and cationicallypolymerizable monomers.

It is preferable to add the basic compound from the viewpoint ofimproving the storage stability of the ink composition. As the basiccompound that can be used in the present invention, a known basiccompound may be used and, for example, a basic inorganic compound suchas an inorganic salt or a basic organic compound such as an amine ispreferably used.

It is preferable to add the polymerization inhibitor from the viewpointof enhancing the storage stability. When the ink composition of thepresent invention is used as an inkjet recording ink composition, it ispreferably heated in the range of 40° C. to 80° C. to thus make it lessviscous and then discharged, and in order to prevent clogging of a headdue to thermal polymerization it is preferable to add a polymerizationinhibitor. The polymerization inhibitor is preferably added at 200 to20,000 ppm relative to the total amount of the ink composition of thepresent invention. Examples of the polymerization inhibitor includehydroquinone, benzoquinone, p-methoxyphenol, TEMPO, TEMPOL, and Alcupferron.

While taking into consideration the ink composition and the inkjetrecording ink composition of the present invention being radiationcuring type ink compositions, it is preferable for them not to containany solvent so that the ink compositions can react quickly and be curedimmediately after landing. However, as long as the curing speed, etc. ofthe ink composition is not affected, a specified solvent may be added.In the present invention, as a solvent, an organic solvent or water maybe used. In particular, the organic solvent may be added in order toimprove the adhesion to a recording medium (a support such as paper).Adding an organic solvent having a high boiling point is effective sincethe problem with VOC can be avoided. The amount of organic solvent ispreferably 0.1 to 5 wt % relative to the total amount of the inkcomposition of the present invention, and more preferably 0.1 to 3 wt %.

As means for preventing the sensitivity from being degraded by a lightblocking effect of the coloring agent, which may be added to the inkcomposition, a combination of a cationically polymerizable compound anda cationic polymerization initiator, a combination of a radicallypolymerizable compound and a radical polymerization initiator, or aradical/cationic hybrid curing ink combining a polymerizable compoundand a polymerization initiator may be employed.

In addition to the above, the ink composition of the present inventionmay contain a known compound as necessary. Examples thereof include asurfactant (cationic, anionic, nonionic, fluorine-based, silicon-based),a leveling additive, a matting agent and, for adjusting film physicalproperties, a polyester resin, polyurethane resin, vinyl resin, acrylicresin, rubber resin, or wax, which may be appropriately selected andadded. Furthermore, in order to improve the adhesion to a recordingmedium such as a polyolefin or PET, a tackifier that does not inhibitpolymerization is preferably added. Specific examples of the tackifierinclude high molecular weight tacky polymers described on pp. 5 and 6 ofJP-A-2001-49200 (e.g. a copolymer formed from an ester of (meth)acrylicacid and an alcohol having an alkyl group with 1 to 20 carbons, an esterof (meth)acrylic acid and an alicyclic alcohol having 3 to 14 carbons,or an ester of (meth)acrylic acid and an aromatic alcohol having 6 to 14carbons), and a low molecular weight tackifying resin having apolymerizable unsaturated bond.

(2) Properties of Ink Composition

The ink composition of the present invention comprises as describedabove (a) the specific onium salt, and (b) a cationically polymerizablecompound, and as necessary (c) a colorant. With regard to thesecomponents, relative to the total solids content of the ink composition,(a) the specific onium salt is preferably 1 to 20 wt %, and morepreferably 2 to 15 wt %, (b) a cationically polymerizable compound ispreferably 10 to 95 wt %, more preferably 30 to 90 wt %, and furthermore preferably 50 to 85 wt %, and in a case which a colorant is used,(c) the colorant is preferably 1 to 10 wt %, and more preferably 2 to 8wt %, and the total wt % including other optional components becomes 100wt %.

When the ink composition thus obtained is used for inkjet recording,while taking into consideration dischargability, the viscosity of theink composition at the discharge temperature (e.g. 25° C. to 80° C.,preferably 25° C. to 40° C., and more preferably 25° C. to 30° C.) ispreferably 5 to 30 mPa·s, and more preferably 7 to 15 mPa·s. Forexample, the ink composition of the present invention has a viscosity atroom temperature (25° C. to 30° C.) of preferably 8 to 300 mPa·s, andmore preferably 10 to 100 mPa·s. With regard to the ink composition ofthe present invention, it is preferable that its component ratio isappropriately adjusted so that the viscosity is in the above-mentionedrange. When the viscosity at room temperature is set to be high, evenwhen a porous recording medium is used, penetration of the ink into therecording medium can be prevented, uncured monomer can be reduced, andthe odor can be reduced. Furthermore, ink spreading when ink dropletshave landed can be suppressed, and as a result there is the advantagethat the image quality is improved. When the viscosity at roomtemperature is set to be low, since it is unnecessary to heat the inkwhen discharging or it is possible to set the heating temperature at arelatively low temperature, there are the advantages that the load oninkjet equipment becomes small and the choice of inkjet heads that canbe used is widened.

The surface tension of the ink composition of the present invention ispreferably 20 to 30 mN/m, and yet more preferably 23 to 28 mN/m. Whenrecording is carried out on various types of recording medium such aspolyolefin, PET, coated paper, and uncoated paper, from the viewpoint ofspread and penetration, it is preferably at least 20 mN/m, and from theviewpoint of wettability it is preferably not more than 30 mN/m.

(3) Inkjet Recording Method and Equipment

The ink composition of the present invention is preferably used forinkjet recording.

An inkjet recording method that can be suitably employed in the presentinvention is explained below.

(3-1) Inkjet Recording Method

The present invention provides a method for forming an image bydischarging the above-mentioned ink composition onto a recording medium(support, recording material, etc.) and curing the ink composition byirradiating the ink composition so discharged onto the recording mediumwith actinic radiation. That is, the present invention relates to aninkjet recording method comprising:

-   (a′) a step of discharging an ink composition onto a recording    medium; and-   (b′) a step of curing the ink composition by irradiating the ink    composition so discharged with actinic radiation,    wherein the ink composition is the ink composition of the present    invention.

The cured ink composition forms an image on the recording medium.

The peak wavelength of the actinic radiation is preferably 200 to 600nm, more preferably 300 to 450 nm, and yet more preferably 350 to 420nm. The output of the actinic radiation is preferably no greater than2,000 mJ/cm², and is more preferably 10 to 2,000 mJ/cm², yet morepreferably 20 to 1,000 mJ/cm², and particularly preferably 50 to 800mJ/cm².

The inkjet recording method of the present invention is explained bytaking as an example a process for producing a lithographic printingplate, the process comprising discharging an ink composition onto thelithographic printing plate so as to form an image.

A process for producing a lithographic printing plate of the presentinvention comprises:

-   (a″) a step of discharging the ink composition of the present    invention onto a hydrophilic support, and-   (b″) a step of irradiating the discharged ink composition with    radiation so as to cure the ink composition, thus forming a    hydrophobic image on the hydrophilic support by curing the ink    composition.    (3-1-1) Hydrophilic Support Used for Lithographic Printing Plate

The lithographic printing plate comprises a support and an image formedon the support.

Conventionally, as the lithographic printing plate, a so-called PS platein which an oleophilic photosensitive resin layer is provided on ahydrophilic support has been widely used. In a process for producingthis PS plate, normally, after a mask exposure (surface exposure) iscarried out via a lith film, non-exposed areas are dissolved and removedto give a desired printing plate. However, in recent years, a techniqueof digitizing image information using a computer by electronicallyprocessing, storing, and outputting the information has becomewidespread, and a new image output system that can be used for the abovetechnique has been desired. In particular, a computer to plate (CTP)technique in which a printing plate is directly produced by scanningaccording to digitized image information with highly coherent light suchas laser light without using a lith film has been developed.

As a system for obtaining a lithographic printing plate that makespossible the above scanning exposure, a process for directly producing alithographic printing plate using an ink composition or an inkjetrecording ink composition can be cited. This process involves obtaininga printing plate having a desired image (preferably a hydrophobic image)by discharging an ink onto a support, and preferably a hydrophilicsupport, using an inkjet system, etc., and exposing this to actinicradiation so as to expose an area with the ink composition or the inkjetrecording ink to light. The ink composition or the inkjet recording inksuitable for such a system is the ink composition or the inkjetrecording ink of the present invention.

The support (recording medium) onto which the ink composition or theinkjet recording ink composition of the present invention is dischargedis not particularly limited, and a dimensionally stable sheet-formsupport may be used. The support is preferably a hydrophilic support.The support used in the lithographic printing plate of the presentinvention is not particularly limited, and a dimensionally stablesheet-form support may be used. It is preferable that a material formingthe support has a hydrophilic surface. Examples of materials forming thesupport include paper, paper laminated with a plastic (e.g.polyethylene, polypropylene, polystyrene, etc.), a metal sheet (e.g.aluminum, zinc, copper, etc.), a plastic film (e.g. cellulose diacetate,cellulose triacetate, cellulose propionate, cellulose butyrate,cellulose acetate butyrate, cellulose nitrate, polyethyleneterephthalate, polyethylene, polystyrene, polypropylene, polycarbonate,polyvinyl acetal, etc.), and paper or plastic film on which theabove-mentioned metal is laminated or vapor-deposited. Preferredexamples of the support include a polyester film and aluminum sheet.Among these, aluminum sheet is particularly preferable since thedimensional stability is good and it is relatively inexpensive.

The aluminum sheet is a pure aluminum sheet, an alloy sheet containingaluminum as a main component and a small amount of a different element,or a thin film of aluminum or an aluminum alloy laminated with aplastic. Examples of the different element contained in the aluminumalloy include silicon, iron, manganese, copper, magnesium, chromium,zinc, bismuth, nickel, and titanium. The content of the differentelement in the alloy is preferably equal to or less than 10 wt %. In thepresent invention, a pure aluminum sheet is preferable, but since it isdifficult to produce completely pure aluminum because of the refiningtechnique, a trace amount of a different element may be contained. Thecomposition of the aluminum sheet is not specified, and a knowngenerally used material may be utilized as appropriate.

The support preferably has a thickness of 0.1 to 0.6 mm, and morepreferably 0.15 to 0.4 mm.

Prior to the aluminum sheet being used, it is preferably subjected to asurface treatment such as a surface roughening treatment or an anodizingtreatment. Surface treatment makes it easy to improve the hydrophilicityand ensure that there is good adhesion between an image recording layerand the support. Prior to the aluminum sheet being subjected to thesurface roughening treatment, it may be subjected as desired to adegreasing treatment using a surfactant, an organic solvent, an aqueousalkaline solution, etc. in order to remove rolling oil on the surface.

The surface roughening treatment for the aluminum sheet surface may becarried out by various types of methods, and examples thereof include amechanical surface roughening treatment, an electrochemical surfaceroughening treatment (a surface roughening treatment involvingdissolving the surface electrochemically), and a chemical surfaceroughening treatment (a surface roughening treatment involvingselectively dissolving the surface chemically).

As a method for the mechanical surface roughening treatment, a knownmethod such as a ball grinding method, a brush grinding method, a blastgrinding method, or a buff grinding method may be used. It is alsopossible to use a transfer method in which an irregular shape istransferred using a roller provided with irregularities in an aluminumrolling stage.

As a method for the electrochemical surface roughening treatment, forexample, a method in which alternating current or direct current isapplied in an electrolyte solution containing an acid such ashydrochloric acid or nitric acid can be cited. It is also possible toemploy a method as described in JP-A-54-63902 in which a mixed acid isused.

The aluminum sheet subjected to a surface roughening treatment issubjected as necessary to an alkali etching treatment using an aqueoussolution of potassium hydroxide, sodium hydroxide, etc.; furthermore,after neutralization, it may be subjected to an anodizing treatment asdesired in order to improve the abrasion resistance.

As an electrolyte that may be used for the anodizing treatment of thealuminum sheet, various types of electrolytes that form a porous oxidefilm may be used. In general, sulfuric acid, hydrochloric acid, oxalicacid, chromic acid, or a mixed acid thereof may be used. Theconcentration of the electrolyte may be determined as appropriateaccording to the type of electrolyte.

Conditions for the anodizing treatment depend on the type of electrolyteused and cannot be specified, but in general the electrolyte solutionconcentration is 1 to 80 wt %, the solution temperature is 5° C. to 70°C., the current density is 5 to 60 A/dm², the voltage is 1 to 100V, andthe electrolysis time is 10 sec. to 5 min. The amount of anodized filmformed is preferably 1.0 to 5.0 g/m², and more preferably 1.5 to 4.0g/m². It is preferable for it to be in this range since good plate lifeand good scratch resistance of a non-image area of a lithographicprinting plate can be obtained.

As the support that can be used in the present invention, a substratethat has been subjected to the above-mentioned surface treatment and hasan anodized film may be used as it is, but in order to further improvethe adhesion to an upper layer, and the hydrophilicity, thecontamination resistance, the thermal insulation, etc., the substratemay appropriately be subjected as necessary to a treatment for enlargingmicropores of the anodized film, a sealing treatment, or a surfacehydrophilization treatment involving immersion in an aqueous solutioncontaining a hydrophilic compound, which are described inJP-A-2001-253181 or JP-A-2001-322365. These enlarging and sealingtreatments are not limited to those described therein, and anyconventionally known methods may be employed.

Sealing Treatment

The sealing treatment may be vapor sealing, a treatment with an aqueoussolution containing an inorganic fluorine compound such as a singletreatment with fluorozirconic acid or a treatment with sodium fluoride,vapor sealing with added lithium chloride, or a sealing treatment withhot water.

Among these, the sealing treatment with an aqueous solution containingan inorganic fluorine compound, the sealing treatment with vapor, andthe sealing treatment with hot water are preferable. Each thereof isexplained below.

<Sealing Treatment With Aqueous Solution Containing Inorganic FluorineCompound>

In the sealing treatment with an aqueous solution containing aninorganic fluorine compound, a metal fluoride can suitably be used asthe inorganic fluorine compound.

Specific examples thereof include sodium fluoride, potassium fluoride,calcium fluoride, magnesium fluoride, sodium fluorozirconate, potassiumfluorozirconate, sodium fluorotitanate, potassium fluorotitanate,ammonium fluorozirconate, ammonium fluorotitanate, potassiumfluorotitanate, fluorozirconic acid, fluorotitanic acid,hexafluorosilicic acid, nickel fluoride, iron fluoride, fluorophosphoricacid, and ammonium fluorophosphate. Among them, sodium fluorozirconate,sodium fluorotitanate, fluorozirconic acid, and fluorotitanic acid arepreferable.

The concentration of the inorganic fluorine compound in the aqueoussolution is preferably at least 0.01 wt % from the viewpoint of sealingof micropores on an anodized coating being carried out sufficiently, andmore preferably at least 0.05 wt %, and it is preferably no greater than1 wt % from the viewpoint of contamination resistance, and morepreferably no greater than 0.5 wt %.

The aqueous solution containing an inorganic fluorine compoundpreferably further contains a phosphate compound. It is preferable for aphosphate compound to be contained since the hydrophilicity of thesurface of the anodized coating improves and the machine developabilityand the contamination resistance can be improved.

Preferred examples of the phosphate compound include phosphates of ametal such as an alkali metal or an alkaline earth metal.

Specific examples thereof include zinc phosphate, aluminum phosphate,ammonium phosphate, diammonium hydrogenphosphate, ammoniumdihydrogenphosphate, potassium dihydrogenphosphate, dipotassiumhydrogenphosphate, calcium phosphate, ammonium sodium hydrogenphosphate,magnesium hydrogenphosphate, magnesium phosphate, ferrous phosphate,ferric phosphate, sodium dihydrogenphosphate, sodium phosphate, disodiumhydrogenphosphate, lead phosphate, diammonium phosphate, calciumdihydrogenphosphate, lithium phosphate, phosphotungstic acid, ammoniumphosphotungstate, sodium phosphotungstate, ammonium phosphomolybdate,sodium phosphomolybdate, sodium phosphite, sodium tripolyphosphate, andsodium pyrophosphate. Among these, sodium dihydrogenphosphate, disodiumhydrogenphosphate, potassium dihydrogenphosphate, and dipotassiumhydrogenphosphate are preferable.

The combination of the inorganic fluorine compound and the phosphatecompound is not particularly limited, but the aqueous solutionpreferably comprises at least sodium fluorozirconate as the inorganicfluorine compound and at least sodium dihydrogen phosphate as thephosphate compound.

The concentration of the phosphate compound in the aqueous solution ispreferably at least 0.01 wt % from the viewpoint of improving machinedevelopability and contamination resistance, and more preferably atleast 0.1 wt %, and it is preferably no greater than 20 wt % from theviewpoint of solubility, and more preferably no greater than 5 wt %.

The proportion of each compound in the aqueous solution is notparticularly limited, but the ratio by weight of the inorganic fluorinecompound and the phosphate compound is preferably 1/200 to 10/1, andmore preferably 1/30 to 2/1.

Furthermore, the temperature of the aqueous solution is preferably atleast 20° C., and more preferably at least 40° C., and it is preferablyno higher than 100° C., and more preferably no higher than 80° C.

Moreover, the pH of the aqueous solution is preferably at least 1, andmore preferably at least 2, and it is preferably no greater than 11, andmore preferably no greater than 5.

A method for the sealing treatment with the aqueous solution containingan inorganic fluorine compound is not particularly limited and, forexample, an immersion method and a spray method may be used. They may beemployed once or a plurality of times, or in a combination of two ormore types.

Among these, the immersion method is preferable. When the treatment iscarried out by the immersion method, the treatment time is preferably atleast 1 sec., and more preferably at least 3 sec., and it is preferablyno greater than 100 sec., and more preferably no greater than 20 sec.

<Sealing Treatment With Steam>

With regard to the sealing treatment with steam, for example, a methodin which an anodized coating is contacted with steam at high pressure ornormal pressure continuously or discontinuously can be cited.

The temperature of the steam is preferably at least 80° C., and morepreferably at least 95° C., and it is preferably no greater than 105° C.

The pressure of the steam is preferably in the range of (atmosphericpressure−50 mmAq) to (atmospheric pressure+300 mmAq) (1.008×10⁵ to1.043×10⁵ Pa).

Furthermore, the time for which the coating is contacted with steam ispreferably at least 1 sec., and more preferably at least 3 sec., and itis preferably no greater than 100 sec., and more preferably no greaterthan 20 sec.

<Sealing Treatment With Hot Water>

With regard to the sealing treatment with hot water, for example, amethod in which an aluminum plate having an anodized coating formedthereon is immersed in hot water can be cited.

The hot water may contain an inorganic salt (e.g. a phosphate) or anorganic salt.

The temperature of the hot water is preferably at least 80° C., and morepreferably at least 95° C., and it is preferably no greater than 100° C.

Furthermore, the time for which immersion in hot water is carried out ispreferably at least 1 sec., and more preferably at least 3 sec., and itis preferably no greater than 100 sec., and more preferably no greaterthan 20 sec.

With regard to a hydrophilization treatment that is used in the presentinvention, there is an alkali metal silicate method, as disclosed inU.S. Pat. Nos. 2,714,066, 3,181,461, 3,280,734, and 3,902,734. In thismethod, a support is immersed in an aqueous solution of sodium silicate,etc., or subjected to electrolysis. In addition, there is a method inwhich a support is treated with potassium fluorozirconate, as describedin JP-B-36-22063, and a method in which a support is treated withpolyvinylphosphonic acid, as described in U.S. Pat. Nos. 3,276,868,4,153,461, and 4,689,272.

In the present invention, it is preferable for the support to have acenter line average roughness of 0.10 to 1.2 μm. It is preferable for itto be in this range since good adhesion to an image recording layer,good plate life, and good contamination resistance can be obtained.

(3-1-2) Step of Discharging Ink Composition Onto Hydrophilic Support

When the ink composition or the inkjet recording ink composition of thepresent invention is discharged onto the surface of the above-mentionedhydrophilic support, the ink composition or the inkjet recording inkcomposition is preferably discharged after being heated to preferably40° C. to 80° C., and more preferably 25° C. to 30° C., so as to reducethe viscosity of the ink composition to preferably 7 to 30 mPa·s, andmore preferably 7 to 20 mPa·s. In particular, it is preferable to usethe ink composition having an ink viscosity at 25° C. of 35 to 500 mPa·ssince a large effect can be obtained. By employing this method, highdischarge stability can be realized. The radiation curing type inkcomposition such as the ink composition of the present inventiongenerally has a viscosity that is higher than that of a normal inkcomposition or a water-based ink used for an inkjet recording ink, andvariation in viscosity due to a change in temperature at the time ofdischarge is large. Viscosity variation in the ink has a large effect onchanges in liquid droplet size and changes in liquid droplet dischargespeed and, consequently, causes the image quality to be degraded. It istherefore necessary to maintain the ink discharge temperature asconstant as possible. In the present invention, the control range forthe temperature is desirably ±5° C. of a set temperature, preferably ±2°C. of the set temperature, and more preferably ±1° C. of the settemperature.

(3-1-3) Step of Curing Ink Composition by Irradiating Discharged InkComposition With Actinic Radiation so as to Form Hydrophobic Image onHydrophilic Support by Curing Ink Composition

The ink composition discharged onto the surface of the hydrophilicsupport is cured by irradiating with actinic radiation. This resultsfrom a sensitizing dye in a polymerization initiation system containedin the above-mentioned ink composition of the present inventionabsorbing actinic radiation, attaining an excited state, and coming intocontact with a polymerization initiator in the polymerization initiationsystem to thus decompose the polymerization initiator, and apolymerizable compound undergoing radical polymerization and beingcured.

The actinic radiation used in this process may include α rays, γ rays,an electron beam, X rays, UV rays, visible light, and IR rays. Althoughit depends on the absorption characteristics of the sensitizing dye, thepeak wavelength of the actinic radiation is, for example, 200 to 600 nm,preferably 300 to 450 nm, and more preferably 350 to 450 nm.Furthermore, in the present invention, the polymerization initiationsystem has sufficient sensitivity for low output actinic radiation. Theoutput of the actinic radiation as irradiation energy is therefore, forexample, 2,000 mJ/cm² or less, and is preferably 10 to 2,000 mJ/cm²,more preferably 20 to 1,000 mJ/cm², and yet more preferably 50 to 800mJ/cm². Moreover, the actinic radiation is applied so that theillumination intensity on the exposed surface is, for example, 10 to2,000 mW/cm², and preferably 20 to 1,000 mW/cm².

The ink composition of the present invention is desirably exposed tosuch actinic radiation for, for example, 0.01 to 120 sec., andpreferably 0.1 to 90 sec.

Irradiation conditions and a basic method for irradiation with actinicradiation are disclosed in JP-A-60-132767. Specifically, a light sourceis provided on either side of a head unit that includes an ink dischargedevice, and the head unit and the light source are made to scan by aso-called shuttle system. Irradiation with actinic radiation is carriedout after a certain time (e.g. 0.01 to 0.5 sec., preferably 0.01 to 0.3sec., and more preferably 0.01 to 0.15 sec.) has elapsed from when theink has landed. By controlling the time from ink landing to irradiationso as to be a minimum in this way, it becomes possible to prevent theink that has landed on a recording medium from spreading before beingcured. Furthermore, since the ink can be exposed before it reaches adeep area of a porous recording medium that the light source cannotreach, it is possible to prevent monomer from remaining unreacted, andas a result the odor can be reduced.

Furthermore, curing may be completed using another light source that isnot driven. WO99/54415 discloses, as an irradiation method, a methodemploying an optical fiber and a method in which a collimated lightsource is incident on a mirror surface provided on a head unit sideface, and a recorded area is irradiated with UV light.

By employing such a recording method, it is possible to maintain auniform dot diameter for landed ink even for various types of recordingmedia having different surface wettability, thereby improving the imagequality. In order to obtain a color image, it is preferable tosuperimpose colors in order from those with a low lightness. Bysuperimposing inks in order from one with low lightness, it is easy forradiation to reach a lower ink, the curing sensitivity is good, theamount of residual monomer decreases, odor is reduced, and animprovement in adhesion can be expected. Furthermore, although it ispossible to discharge all colors and then expose them at the same time,it is preferable to expose one color at a time from the viewpoint ofpromoting curing.

In this way, the above-mentioned ink composition of the presentinvention is cured by irradiation with actinic radiation to thus form ahydrophobic image on the surface of the hydrophilic support.

(3-2) Inkjet Recording Device

The inkjet recording device used in the present invention is notparticularly restricted, and a commercial inkjet recording device may beused. That is, in the present invention, recording on a recording mediummay be carried out using a commercial inkjet recording device.

The inkjet recording device that can be used in the present invention isequipped with, for example, an ink supply system, a temperature sensor,and an actinic radiation source.

The ink supply comprises, for example, a main tank containing the inkcomposition of the present invention, a supply pipe, an ink supply tankimmediately before an inkjet head, a filter, and a piezo system inkjethead. The piezo system inkjet head may be driven so as to discharge amultisize dot of 1 to 100 pL, and preferably 8 to 30 pL, at a resolutionof 320×320 to 4,000×4,000 dpi, preferably 400×400 to 1,600×1,600 dpi,and more preferably 720×720 dpi. Here, dpi referred to in the presentinvention means the number of dots per 2.54 cm.

As described above, since it is desirable for the radiation curing typeink to be discharged at a constant temperature, a section from the inksupply tank to the inkjet head is thermally insulated and heated. Amethod of controlling temperature is not particularly limited, but it ispreferable to provide, for example, temperature sensors at a pluralityof pipe section positions, and control heating according to the ink flowrate and the temperature of the surroundings. The temperature sensorsmay be provided on the ink supply tank and in the vicinity of the inkjethead nozzle. Furthermore, the head unit that is to be heated ispreferably thermally shielded or insulated so that the device main bodyis not influenced by the temperature of the outside air. In order toreduce the printer start-up time required for heating, or in order toreduce the thermal energy loss, it is preferable to thermally insulatethe head unit from other sections and also to reduce the heat capacityof the entire heated unit.

As an actinic radiation source, a mercury lamp, a gas/solid laser, etc.are mainly used, and for UV photocuring inkjet a mercury lamp and ametal halide lamp are widely known. However, from the viewpoint ofprotection of the environment, there has recently been a strong desirefor mercury not to be used, and replacement by a GaN semiconductor UVlight emitting device is very useful from industrial and environmentalviewpoints. Furthermore, LEDs (UV-LED) and LDs (UV-LD) have smalldimensions, long life, high efficiency, and low cost, and their use as aphotocuring inkjet light source can be expected.

Furthermore, light-emitting diodes (LED) and laser diodes (LD) may beused as the source of actinic radiation. In particular, when a UV raysource is needed, a UV-LED or a UV-LD may be used. For example, NichiaCorporation has marketed a violet LED having a wavelength of the mainemission spectrum of between 365 nm and 420 nm. Furthermore, when ashorter wavelength is needed, U.S. Pat. No. 6,084,250 discloses an LEDthat can emit actinic radiation whose wavelength is centered between 300nm and 370 nm. Furthermore, another violet LED is available, andirradiation can be carried out with radiation of a different UVbandwidth. The actinic radiation source particularly preferable in thepresent invention is a UV-LED, and a UV-LED having a peak wavelength at350 to 420 nm is particularly preferable.

The maximum illumination intensity of the LED on a recording medium ispreferably 10 to 2,000 mW/cm², more preferably 20 to 1,000 mW/cm², andparticularly preferably 50 to 800 mJ/cm².

In accordance with the present invention, it is possible to provide anink composition that cures with high sensitivity when exposed toradiation, can form a high quality image, has excellent adhesion to arecording medium, and has good storage stability, and an inkjetrecording method employing the ink composition.

Furthermore, a printed material obtained using the ink compositionhaving excellent storage stability, and being capable of curing withhigh sensitivity when exposed to radiation has high image quality andexcellent strength for an image area. Similarly, in accordance with useof the ink composition of the present invention, there is exhibited theeffect that a lithographic printing plate having a long plate life andhigh image quality can be produced based on digital data.

EXAMPLES

The present invention is explained in further detail by reference toExamples and Comparative Examples. However, the present invention shouldnot be construed as being limited to these Examples.

Preparation of Pigment Dispersions

Yellow, magenta, cyan, and black pigment dispersions 1 were prepared inaccordance with the method described below. Dispersion conditions wereappropriately adjusted using a known dispersing machine so that theaverage particle size of pigment particles was in the range of 0.2 to0.3 μm, and they were subsequently filtered using a filter whileheating.

Yellow pigment dispersion 1 CI Pigment Yellow 13 20 parts by weightPolymeric dispersant 20 parts by weight (Solsperse series manufacturedby Zeneca) Diethylene glycol divinyl ether 60 parts by weight(manufactured by BASF)

Magenta pigment dispersion 1 CI Pigment Red 57:1 20 parts by weightPolymeric dispersant 20 parts by weight (Solsperse series manufacturedby Zeneca) Diethylene glycol divinyl ether 60 parts by weight(manufactured by BASF)

Cyan pigment dispersion 1 CI Pigment Blue 15:3 20 parts by weightPolymeric dispersant 20 parts by weight (Solsperse series manufacturedby Zeneca) Diethylene glycol divinyl ether 60 parts by weight(manufactured by BASF)

Black pigment dispersion 1 CI Pigment black 7 20 parts by weightPolymeric dispersant 20 parts by weight (Solsperse series manufacturedby Zeneca) Diethylene glycol divinyl ether 60 parts by weight(manufactured by BASF)Preparation of Inks

Example 1

Yellow ink 1 Yellow pigment dispersion 1  5 parts by weight Compound Abelow (counteranion volume:  6 parts by weight 2,091 Å³) Sensitizingdye: 9,10-dibutoxyanthracene  3 parts by weight Polymerizable compoundsMonomer: 3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weightepoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCBCo., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane (OXT- 45 parts byweight 221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1(Amine-1 below)  1 parts by weight

Compound A

Amine-1 (Compound described in JP-A-2003-305839)

Magenta ink 1 Magenta pigment dispersion 1 5 parts by weight Compound A6 parts by weight Polymerizable compounds Monomer:3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weightepoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCBCo., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight(OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 partby weight

Cyan ink 1 Cyan pigment dispersion 1 5 parts by weight Compound A 6parts by weight Polymerizable compounds Monomer:3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weightepoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCBCo., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight(OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 partby weight

Black ink 1 Black pigment dispersion 1 5 parts by weight Compound A 6parts by weight Polymerizable compounds Monomer:3,4-epoxycyclohexylmethyl-3′,4′- 40 parts by weightepoxycyclohexanecarboxylate (Celloxide 2021A: manufactured by Daicel-UCBCo., Ltd.) Monomer: 3,7-bis(3-oxetanyl)-5-oxanonane 48 parts by weight(OXT-221: manufactured by Toagosei Co., Ltd.) Basic compound: a-1 1 partby weight

Inks 1 of each color prepared above were filtered using a filter havingan absolute filtration accuracy of 2 μm to give Inks 1 of each color.

Inkjet Image Recording

Subsequently, recording was carried out on a recording medium using acommercial inkjet recording device having a piezo system inkjet nozzle.The ink supply system comprised a main tank, a supply pipe, an inksupply tank immediately before an inkjet head, a filter, and a piezosystem inkjet head, and a section from the ink supply tank to the inkjethead was thermally insulated and heated. Temperature sensors wereprovided on the ink supply tank and in the vicinity of the nozzle of theinkjet head, and the temperature was controlled so that the nozzlesection was always at 70° C. ±2° C. The piezo system inkjet head wasdriven so as to discharge a multisize dot of 8 to 30 pL at a resolutionof 720×720 dpi. The exposure system, the main scanning speed, and thedischarge frequency were adjusted so that, after landing, UV-A light wasfocused to give an exposure area illumination intensity of 100 mW/cm²,and irradiation started 0.1 sec. after the ink landed on the recordingmedium. Furthermore, the exposure time was made variable, and exposureenergy was applied. Here, dpi referred to in the present inventiondenotes the number of dots per 2.54 cm.

The inks of each color prepared above were discharged in the orderblack, cyan, magenta, and yellow at an environmental temperature of 25°C., and each color was irradiated with UV rays using a VZero 085 metalhalide lamp manufactured by Integration Technology. As an energy levelthat could completely cure the inks so that tackiness disappeared whentouched by hand, the total exposure energy per color was 100 mJ/cm² forall the colors. As recording media, a grained aluminum support, atransparent biaxially stretched polypropylene film whose surface hadbeen treated so as to impart printability, a soft vinyl chloride sheet,a cast coat paper, and a commercial recycled paper were used, each colorimage was recorded, and an image having high resolution without dotspreading was obtained in all cases. Furthermore, for high qualitypaper, the ink did not penetrate to the reverse side, the ink wassufficiently cured, and there was hardly any odor due to unreactedmonomer. Moreover, the ink recorded on the film had sufficientflexibility, the ink did not crack when bent, and there was no problemin an adhesion test involving peeling with Sellotape (registeredtrademark).

Examples 2 to 9

Preparation of Inks

Magenta inks 2 to 9 were prepared in accordance with the methoddescribed below.

Magenta inks 2 to 9

Magenta inks 2 to 9 were prepared in the same manner as for the abovemagenta ink 1 except that the specific onium salt and other additionalcomponents were changed to those described in the table below.

TABLE 2 Volume of counteranion Magenta of specific ink No. (a) Specificonium salt onium salt (a) Added component Ex. 2 2

1,514 Ex. 3 3

2,091 Ex. 4 4

2,091 Ex. 5 5

2,091 Ex. 6 6

1,298 Ex. 7 7

3,897 Ex. 8 8

1,020 Ex. 9 9

2,091 9,10-Dibutoxyanthracene3 parts by weight

Example 10: Magenta ink 10 Magenta pigment dispersion 1 5 parts byweight The above compound A 6 parts by weight Polymerizable compoundsCationically polymerizable monomer: 3,4- 30 parts by weightepoxycyclohexylmethyl-3′,4′- epoxycyclohexanecarboxylate (Celloxide2021A: manufactured by Daicel-UCB Co., Ltd.) Cationically polymerizablemonomer: 3,7-bis 48 parts by weight (3-oxetanyl)-5-oxanonane (OXT-221:manufactured by Toagosei Co., Ltd.) Radically polymerizable monomer:1,6-hexanediol 10 parts by weight diacrylate Basic compound: a-1 1 partby weight

Magenta ink 10 prepared as above was filtered using a filter having anabsolute filtration accuracy of 2 μm to give magenta ink 10.

Comparative Examples 1 to 4

Preparation of Ink

Magenta inks 11 to 14 were prepared in the same manner as for the abovemagenta ink 1 except that the cationic polymerization initiator waschanged to those described in the table below.

TABLE 3 Volume of counteranion of specific Magenta onium salt (a) inkNo. (a) Specific onium salt (Å³) Comp.Ex. 1 11

95 Comp.Ex. 2 12

127 Comp.Ex. 3 13

537 Comp.Ex. 4 14

783

Magenta inks 11 to 14 prepared as above were filtered using a filterhaving an absolute filtration accuracy of 2 μm to give Magenta inks 1 1to 14.

Example 11

Magenta ink 15 was prepared in the same manner as for magenta ink 1except that Compound M-1 below (oxidation potential +1.37 V) was used asan oil-soluble dye instead of CI pigment red 57:1, and filtration wascarried out using a filter having an absolute filtration accuracy of 2μm.

Example 12

Magenta ink 16 was prepared in the same manner as for magenta ink 1except that Compound M-2 below (oxidation potential +0.94 V) was used asan oil-soluble dye instead of CI pigment red 57:1, and filtration wascarried out using a filter having an absolute filtration accuracy of 2μm.

Inkjet Image Recording

Magenta images were produced in the same manner as in the methoddescribed in Example 1 using the magenta inks 2 to 16 prepared as above.

Example 13

A magenta image was formed using Magenta ink 9 by the same method as inExample 1 except that a UV light-emitting diode (UV-LED) was usedinstead of the VZero 085 metal halide lamp manufactured by IntegrationTechnology.

In this embodiment, an NCCU033 manufactured by Nichia Corporation wasused as the UV-LED. The LED emits UV light at a wavelength of 365 nmfrom 1 chip, and by applying a current of about 500 mA, light of about100 mW is emitted from the chip. A plurality thereof were aligned atintervals of 7 mm to give a power of 0.3 W/cm² on the surface of arecording medium (hereinafter, also called a medium). The time fromlanding to irradiation and the exposure time can be varied by thetransport speed of the medium and the distance between a head and theLED in the transport direction. In this embodiment, irradiation wascarried out about 0.5 sec. after landing.

The exposure energy on the medium was adjustable in the range of 0.01 to15 J/cm² by setting the distance from the medium and the transportspeed.

Comparative Example 5

Magenta ink 17 was prepared in the same manner as for magenta ink 14except that 3 parts by weight of 9,10-dibutoxyanthracene was added, andfiltration was carried out using a filter having an absolute filtrationaccuracy of 2 μm.

A magenta image was produced in the same manner as in the method ofExample 13 using magenta ink 17 prepared as above.

Evaluation of Inkjet Image

The images thus formed were subjected to evaluation in accordance withthe methods described below in terms of sensitivity required for curing,penetration into commercial recycled paper, ink spreading on a grainedaluminum support, adhesion, plate life, and storage stability.

Curing Sensitivity Measurement

The exposure energy intensity (mJ/cm²) when a feeling of tackinessdisappeared on the image surface after irradiation with ultraviolet rayswas defined as the curing sensitivity. The smaller the value, the higherthe sensitivity.

Evaluation of Penetration into Commercial Recycled Paper

Images printed on commercial recycled paper were evaluated in terms ofpenetration in accordance with the criteria below.

-   Good: Hardly any penetration, no odor of residual monomer.-   Fair: Slight penetration, slight odor of residual monomer.-   Poor: Ink obviously penetrated to the reverse side, and strong odor    of residual monomer.    Evaluation of Ink Spread on Grained Aluminum Support

With respect to an image printed on a grained aluminum support, inkspread was evaluated in accordance with the criteria below.

-   Good: no spread between adjacent dots.-   Fair: dots slightly spread.-   Poor: dots spread and image was obviously blurred.    Evaluation of Adhesion to Grained Aluminum Support

With regard to the printed images formed above, a completely undamagedsample and a sample whose printed surface was crosshatched with 11 cutsin both lengthwise and widthwise directions at intervals of 1 mm inaccordance with JIS K 5400 to give 100 of 1 mm squares were prepared,Sellotape was affixed to the printed surface of each sample and peeledoff quickly at an angle of 90 degrees, and the condition of theremaining printed image or squares that had not been peeled off wasevaluated in accordance with the criteria below.

-   Good: printed image was not peeled off at all in the crosshatch    test.-   Fair: the ink was slightly peeled off in the crosshatch test, but    unless the ink surface was damaged little was peeled off.-   Poor: easily peeled off by Sellotape (registered trademark) under    both conditions.    Evaluation of Plate Life

An image printed on a grained aluminum support prepared above was usedas a printing plate, printing was carried out using a Heidel KOR-Dmachine, and a relative comparison of the number of prints completed wasused as an index for the plate life (the number obtained for Example 1was defined as 100). The larger the number, the longer the plate life,which is preferable.

Storage Stability Evaluation

After storing the prepared ink at 75%RH and 60° C. for 3 days, the inkviscosity at the. discharge temperature was measured, and an increase inthe ink viscosity was expressed as a viscosity ratio (afterstorage/before storage). When the viscosity was unchanged and the ratiowas close to 1.0, the storage stability was good, and if the ratioexceeded 1.5, clogging might undesirably be caused during discharge.

These evaluation results are shown in Table 4.

TABLE 4 Ma- Curing genta sensi- Storage ink tivity Pene- stabil- Ink Ad-Plate No. (mJ/cm²) tration ity spread hesion life Ex. 1 1 100 Good 1.1Good Good 100 Ex. 2 2 90 Good 1.1 Good Good 110 Ex. 3 3 110 Good 1.1Good Good 90 Ex. 4 4 80 Good 1.3 Good Good 120 Ex. 5 5 110 Good 1.1 GoodGood 90 Ex. 6 6 110 Good 1.1 Good Good 95 Ex. 7 7 120 Good 1.1 Good Good85 Ex. 8 8 120 Good 1.1 Good Good 85 Ex. 9 9 70 Good 1.2 Good Good 150Ex. 10 10 90 Good 1.1 Good Good 110 Ex. 11 15 90 Good 1.1 Good Good 110Ex. 12 16 80 Good 1.1 Good Good 120 Ex. 13 9 70 Good 1.2 Good Good 150Comp. 11 250 Fair 1.1 Fair Fair 60 Ex. 1 Comp. 12 200 Good 1.1 Good Fair70 Ex. 2 Comp. 13 200 Good 1.1 Good Fair 70 Ex. 3 Comp. 14 200 Good 1.1Good Fair 70 Ex. 4 Comp. 17 180 Good 1.2 Good Fair 75 Ex. 5

It is clear from Table 4 that the ink composition of the presentinvention comprising (a) an onium salt, the counteranion thereof havinga volume of 1,000 Å³ or greater, and (b) a cationically polymerizablecompound has high sensitivity toward radiation, can form a high qualityimage in terms of image formation properties on paper, has good storagestability, and can form a high quality image with high plate life whenapplied to the production of a printing plate.

1. An ink composition comprising: (a) an onium salt, a counteranion thereof having a volume of 1,000 Å³ or greater; and (b) a cationically polymerizable compound, wherein the counteranion of the onium salt is an anion selected from the group consisting of An-1 to An-12 below,


2. The ink composition according to claim 1, wherein the counteranion of the onium salt has a volume of 1,000 to 5,000 Å³.
 3. The ink composition according to claim 1, wherein the counteranion of the onium salt is an organoboron anion.
 4. The ink composition according to claim 1, wherein the counteranion of the onium salt is an anion in which at least one perfluoroalkyl group is bonded directly to an anionic atom or is bonded to the anionic atom via an unsaturated bond or an aromatic ring.
 5. The ink composition according to claim 1, wherein a countercation of the onium salt is a cation selected from the group consisting of iodonium, sulfonium, phosphonium, diazonium, ammonium, pyridinium, quinolinium, acridinium, oxonium, selenonium, and arsonium.
 6. The ink composition according to claim 1, wherein the ink composition comprises a colorant.
 7. The ink composition according to claim 6, wherein the colorant is a pigment or an oil-soluble dye.
 8. The ink composition according to claim 7, wherein the oil-soluble dye has an oxidation potential of 1.0 V (vs SCE) or greater.
 9. The ink composition according to claim 1, wherein it is for inkjet recording.
 10. An inkjet recording method comprising: (a′) a step of discharging an ink composition onto a recording medium; and (b′) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation, wherein the ink composition is the ink composition according to claim
 1. 11. The inkjet recording method according to claim 10, wherein the actinic radiation is ultraviolet radiation emitted by a light emitting diode that has a light emission peak wavelength in the range of 350 to 420 nm and generates ultraviolet radiation whose maximum illumination intensity on the surface of a recording medium is 10 to 2,000 mW/cm².
 12. A process for producing a lithographic printing plate, the process comprising: (a″) a step of discharging the ink composition according to claim 1 onto a hydrophilic support; and (b″) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support from the cured ink composition.
 13. The ink composition according to claim 1, wherein the countercation of the onium salt is a cation having structures represented by Formulae (I) to (VI) below,

wherein R¹ to R³ independently denote an aryl group, R⁴ to R⁶ independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, or a heterocyclic group, R⁷ to R¹¹ independently denote an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cyclic hydrocarbon group, a heterocyclic group, an alkoxy group, or an aryloxy group, and R¹² to R¹⁷ independently denote a hydrogen atom, a halogen atom, or a monovalent organic group. 